formation of hydrogen peroxide as a compound containing two aqueous
residues (OH)(OH), then on the basis of this law all hydrocarbons
ought to be derived from methane, CH_{4}, as being the simplest
hydrocarbon.[36] The increase in complexity of a molecule of methane
is brought about by the faculty of mutual combination which exists in
the atoms of carbon, and, as a consequence of the most detailed study
of the subject, much that might have been foreseen and conjectured
from the law of substitution has been actually brought about in such
a manner as might have been predicted, and although this subject on
account of its magnitude really belongs, as has been already stated,
to the sphere of organic chemistry, it has been alluded to here in
order to show, although only in part, the best investigated example of
the application of the law of substitution. According to this law, a
molecule of methane, CH_{4}, is capable of undergoing substitution in
the four following ways:--(1) Methyl substitution, when the radicle,
equivalent to hydrogen, called _methyl_ CH_{3}, replaces hydrogen.
In CH_{4} this radicle is combined with H and therefore can replace
it, as (OH) replaces H because with it it gives water; (2) methylene
substitution, or the exchange between H_{2} and CH_{2} (this radicle
is called methylene), is founded on a similar division of the molecule
CH_{4} into two equivalent parts, H_{2} and CH_{2}; (3) acetylene
substitution, or the exchange between CH on the one hand and H_{3}
on the other; and (4) carbon substitution--that is, the substitution
of H_{4} by an atom of carbon C, which is founded on the law of
substitution just as is the methyl substitution. These four cases of
substitution render it possible to understand the principal relations of
the hydrocarbons. For instance, the _law of even numbers_ is seen from
the fact that in all the cases of substitution mentioned the hydrogen
atoms increase or decrease by an even number; but as in CH_{4} they are
likewise even, it follows that no matter how many substitutions are
effected there will always be obtained an even number of hydrogen atoms.
When H is replaced by CH_{3} there is an increase of CH_{2}; when H_{2}
is replaced by CH_{2} there is no increase of hydrogen; in the acetylene
substitution CH replaces H_{3}, therefore there is an increase of C and
a decrease of H_{2}; in the carbon substitution there is a decrease
of H_{4}. In a similar way the _law of limit_ may be deduced as a
corollary of the law of substitution. For the largest possible quantity
of hydrogen is introduced by the methyl substitution, since it leads
to the addition of CH_{2}; starting from CH_{4} we obtain C_{2}H_{6},
C_{3}H_{8}, and in general, C_{_n_}H_{2_n_+2}, and these contain
the greatest possible amount of hydrogen. Unsaturated hydrocarbons,
containing less hydrogen, are evidently only formed when the increase
of the new molecule derived from methane proceeds from one of the
other forms of substitution. When the methyl substitution alone takes
place in methane, CH_{4}, it is evident that the saturated hydrocarbon
formed is C_{2}H_{6} or (CH_{3})(CH_{3}).[37] This is called _ethane_.
By means of the methylene substitution alone, _ethylene_, C_{2}H_{4},
or (CH_{2})(CH_{2}) may be directly obtained from CH_{4}, and by the
acetylene substitution C_{2}H_{2} or (CH)(CH), or _acetylene_, both the
latter being unsaturated hydrocarbons. Thus we have all the possible
hydrocarbons with two atoms of carbon in the molecule, C_{2}H_{6},
ethane, C_{2}H_{4}, ethylene, and C_{2}H_{2}, acetylene. But in them,
according to the law of substitution, the same forms of substitution may
be repeated--that is, the methyl, methylene, acetylene, and even carbon
substitutions (because C_{2}H_{6} will still contain hydrogen when C
replaces H_{4}) and therefore further substitutions will serve as a
source for the production of a fresh series of saturated and unsaturated
hydrocarbons, containing more and more carbon in the molecule and,
in the case of the acetylene substitution and carbon substitution,
containing less and less hydrogen. Thus _by means of the law of
substitution we can foresee_ not only the limit C_{_n_}H_{2_n_+2},
but an unlimited number of unsaturated hydrocarbons, C_{_n_}H_{2_n_},
C_{_n_}H_{2_n_-2} ... C_{_n_}H_{2(_n-m_)}, where _m_ varies from 0
to _n_-1,[38] and where _n_ increases indefinitely. From these facts
not only does the existence of a multitude of polymeric hydrocarbons,
differing in molecular weight, become intelligible, but it is also
seen that there is a possibility of cases of isomerism with the same
molecular weight. This _polymerism_ so common to hydrocarbon compounds
is already apparent in the first unsaturated series C_{_n_}H_{2_n_},
because all the terms of this series C_{2}H_{4}, C_{3}H_{6}, C_{4}H_{8}
... C_{30}H_{60} ... have one and the same composition CH_{2}, but
different molecular weights, as has been already explained in Chapter
VII. The differences in the vapour density, boiling points, and
melting points, of the quantities entering into reactions,[39] and the
methods of preparation[40] also so clearly tally with the conception
of polymerism, that this example will always be the clearest and most
conclusive for the illustration of polymerism and molecular weight.
Such a case is also met with among other hydrocarbons. Thus benzene,
C_{6}H_{6}, and cinnamene, C_{8}H_{8}, correspond with the composition
of acetylene or to a compound of the composition CH.[41] The first boils
at 81°, the second at 144°; the specific gravity of the first is 0·899;
that of the second, 0·925, at 0°--that is, here also the boiling point
rises with the increase of molecular weight, and so also, as might be
expected, does the density.
[36] Methylene, CH_{2}, does not exist. When attempts are made to
obtain it (for example, by removing X_{2} from CH_{2}X_{2}),
C_{2}H_{4} or C_{3}H_{6} are produced--that is to say, it
undergoes polymerisation.
[37] Although the methods of formation and the reactions connected with
hydrocarbons are not described in this work, because they are
dealt with in organic chemistry, yet in order to clearly show
the mechanism of those transformations by which the carbon
atoms are built up into the molecules of the carbon compounds,
we here give a general example of reactions of this kind. From
marsh gas, CH_{4}, on the one hand the substitution of chlorine
or iodine, CH_{3}Cl, CH_{3}I, for the hydrogen may be effected,
and on the other hand such metals as sodium may be substituted
for the hydrogen, _e.g._ CH_{3}Na. These and similar products of
substitution serve as a means of obtaining other more complex
substances from given carbon compounds. If we place the two
above-named products of substitution of marsh gas (metallic and
haloid) in mutual contact, the metal combines with the halogen,
forming a very stable compound--namely, common salt, NaCl, and
the carbon groups which were in combination with them separate in
mutual combination, as shown by the equation:
CH_{3}Cl + CH_{3}Na = NaCl + C_{2}H_{6}.
This is the most simple example of the formation of a complex
hydrocarbon from these radicles. The cause of the reaction must
be sought for in the property which the haloid (chlorine) and
sodium have of entering into mutual combination.
[38] When _m_ = _n_-1, we have the series C_{_n_}H_{2}. The lowest
member is acetylene, C_{2}H_{2}. These are hydrocarbons
containing a minimum amount of hydrogen.
[39] For instance, ethylene, C_{2}H_{4}, combines with Br_{2}, HI,
H_{2}SO_{4}, as a whole molecule, as also does amylene,
C_{5}H_{10}, and, in general, C_{_n_}H_{2_n_}.
[40] For instance, ethylene is obtained by removing the water from
ethyl alcohol, C_{2}H_{5}(OH), and amylene, C_{5}H_{10}, from
amyl alcohol, C_{5}H_{11}(OH), or in general C_{_n_}H_{2_n_},
from C_{_n_}H_{2_n_+1}(OH).
[41] Acetylene and its polymerides have an empirical composition CH,
ethylene and its homologues (and polymerides) CH_{2}, ethane
CH_{3}, methane CH_{4}. This series presents a good example of
the law of multiple proportions, but such diverse proportions
are met with between the number of atoms of the carbon and
hydrogen in the hydrocarbons already known that the accuracy of
Dalton's law might be doubted. Thus the substances C_{30}H_{62}
and C_{30}H_{60} differ so slightly in their composition by
weight as to be within the limits of experimental error, but
their reactions and properties are so distinct that they can be
distinguished beyond a doubt. Without Dalton's law chemistry
could not have been brought to its present condition, but it
cannot alone express all those gradations which are quite clearly
understood and predicted by the law of Avogadro-Gerhardt.
Cases of isomerism in the restricted sense of the word--that is,
when with an identity of composition and of molecular weight, the
properties of the substances are different--are very numerous among
the hydrocarbons and their derivatives. Such cases are particularly
important for the comprehension of molecular structure and they also,
like the polymerides, may be predicted from the above-mentioned
conceptions, expressing the principles of the structure of the carbon
compounds[42] based on the law of substitution. According to it, for
example, it is evident that there can be no isomerism in the cases
of the saturated hydrocarbons C_{2}H_{6} and C_{3}H_{8}, because the
former is CH_{4}, in which methyl has taken the place of H, and as
all the hydrogen atoms of methane must be supposed to have the same
relation to the carbon, it is all the same which of them be subjected
to the methyl substitution--the resulting product can only be ethane,
CH_{3}CH_{3};[43] the same argument also applies in the case of propane,
CH_{3}CH_{2}CH_{3}, where one compound only can be imagined. It is to
be expected, however, that there should be two butanes, C_{4}H_{10}, and
this is actually the case. In one, methyl may be considered as replacing
the hydrogen of one of the methyls, CH_{3}CH_{2}CH_{2}CH_{3}; and in
the other CH_{3} may be considered as substituted for H in /CH_{3}
CH_{2}, and there it will consist of CH_{3}CH. The latter may \CH_{3}
also be regarded as methane in which three of hydrogen are exchanged
for three of methyl. On going further in the series it is evident that
the number of possible isomerides will be still greater, but we have
limited ourselves to the simplest examples, showing the possibility and
actual existence of isomerides. C_{2}H_{4} and CH_{2}CH_{2} are, it is
evident, identical; but there ought to be, and are, two hydrocarbons of
the composition C_{3}H_{6}, propylene and trimethylene; the first is
ethylene, CH_{2}CH_{2}, in which one atom of hydrogen is exchanged for
methyl, CH_{2}CHCH_{3}, and trimethylene is ethane, CH_{3}CH_{3}, with
the substitution of methylene for two hydrogen atoms from two methyl
groups--that /CH_{2} is, CH_{2},[44] where the methylene introduced
is united to both \CH_{2} the atoms of carbon in CH_{3}CH_{3}. It is
evident that the cause of isomerism here is, on the one hand, the
difference of the amount of hydrogen in union with the particular
atoms of carbon, and, on the other, the different connection between
the several atoms of carbon. In the first case they may be said to
be chained together (more usually to form an 'open chain'), and in
the second case, to be locked together (to form a 'closed chain'
or 'ring'). Here also it is easily understood that on increasing
the quantity of carbon atoms the number of possible and existing
isomerides will greatly increase. If, at the same time, in addition
to the substitution of one of the radicles of methane for hydrogen a
further exchange of part of the hydrogen for some of the other groups
of elements X, Y ... occurs, the quantity of possible isomerides still
further increases in a considerable degree. For instance, there are
even two possible isomerides for the derivatives of ethane, C_{2}H_{6}:
if two atoms of the hydrogen be exchanged for X_{2}, one will have
the ethylene structure, CH_{2}XCH_{2}X, and the other an ethylidene
structure, CH_{3}CHX_{2}; such are, for instance, ethylene chloride,
CH_{2}ClCH_{2}Cl, and ethylidene chloride, CH_{3}CHCl_{2}. And as in the
place of the first atom of hydrogen not only metals may be substituted,
but Cl, Br, I, OH (the water radicle), NH_{2} (the ammonia radicle),
NO_{2} (the radicle of nitric acid), &c., so also in exchange for two
atoms of hydrogen O, NH, S, &c., may be substituted; hence it will be
understood that the quantity of isomerides is sometimes very great. It
is impossible here to describe how the isomerides are distinguished
from each other, in what reactions they occur, how and when one changes
into another, &c.; for this, taken together with the description of
the hydrocarbons already known, and their derivatives, forms a very
extensive and very thoroughly investigated branch of chemistry, called
_organic chemistry_. Enriched with a mass of closely observed phenomena
and strictly deduced generalisations, this branch of chemistry has
been treated separately for the reason that in it the hydrocarbon
groups are subjected to transformations which are not met with in such
quantity in dealing with any of the other elements or their hydrogen
compounds. It was important for us to show that notwithstanding the
great variety of the hydrocarbons and their products,[45] they are all
of them governed by the law of substitution, and referring our readers
for detailed information to works on organic chemistry, we will limit
ourselves to a short exposition of the properties of the two simplest
unsaturated hydrocarbons: ethylene, CH_{2}CH_{2}, and acetylene, CHCH,
and a short acquaintance with petroleum as the natural source of a mass
of hydrocarbons. _Ethylene, or olefiant gas_, C_{2}H_{4}, is the lowest
known member of the unsaturated hydrocarbon series of the composition
C_{_n_}H_{2_n_}. As in composition it is equal to two molecules of marsh
gas deprived of two molecules of hydrogen, it is evident that it might
be, and it actually can be, produced, although but in small quantities,
together with hydrogen, by heating marsh gas. On being heated, however,
olefiant gas splits up, first into acetylene and methane (3C_{2}H_{4} =
2C_{2}H_{2} + 2CH_{4}, Lewes, 1894), and at a higher temperature into
carbon and hydrogen; and therefore in those cases where marsh gas is
produced by heating, olefiant gas, hydrogen, and charcoal will also be
formed, although only in small quantities. The lower the temperature at
which complex organic substances are heated, the greater the quantity
of olefiant gas found in the gases given off; at a white heat it is
entirely decomposed into charcoal and marsh gas. If coal, wood, and more
particularly petroleum, tars, and fatty substances, are subjected to dry
distillation, they give off illuminating gas, which contains more or
less olefiant gas.
[42] The conception of the structure of carbon compounds--that is, the
expression of those unions and correlations which their atoms
have in the molecules--was for a long time limited to the
representation that organic substances contained complex
radicles (for instance, ethyl C_{2}H_{5}, methyl CH_{3}, phenyl
C_{6}H_{5}, &c.); then about the year 1840 the phenomena
of substitution and the correspondence of the products of
substitution with the primary bodies (nuclei and types) were
observed, but it was not until about the year 1860 and later when
on the one hand the teaching of Gerhardt about molecules was
spreading, and on the other hand the materials had accumulated
for discussing the transformations of the simplest hydrocarbon
compounds, that conjectures began to appear as to the mutual
connection of the atoms of carbon in the molecules of the complex
hydrocarbon compounds. Then Kekulé and A. M. Butleroff began to
formulate the connection between the separate atoms of carbon,
regarding it as a quadrivalent element. Although in their
methods of expression and in some of their views they differ
from each other and also from the way in which the subject is
treated in this work, yet the essence of the matter--namely,
the comprehension of the causes of isomerism and of the union
between the separate atoms of carbon--remains the same. In
addition to this, starting from the year 1870, there appears a
tendency which from year to year increases to discover the actual
spacial distribution of the atoms in the molecules. Thanks to the
endeavours of Le-Bel (1874), Van't Hoff (1874), and Wislicenus
(1887) in observing cases of isomerism--such as the effect of
different isomerides on the direction of the rotation of the
plane of polarisation of light--this tendency promises much
for chemical mechanics, but the details of the still imperfect
knowledge in relation to this matter must be sought for in
special works devoted to organic chemistry.
[43] Direct experiment shows that however CH_{3}X is prepared (where
X = for instance Cl, &c.) it is always one and the same
substance. If, for example, in CX_{4}, X is gradually replaced by
hydrogen until CH_{3}X is produced, or in CH_{4}, the hydrogen by
various means is replaced by X, or else, for instance, if CH_{3}X
be obtained by the decomposition of more complex compounds, the
same product is always obtained.
This was shown in the year 1860, or thereabout, by many methods,
and is the fundamental conception of the structure of hydrocarbon
compounds. If the atoms of hydrogen in methyl were not absolutely
identical in value and position (as they are not, for instance,
in CH_{3}CH_{2}CH_{3} or CH_{3}CH_{2}X), then there would be as
many different forms of CH_{3}X as there were diversities in the
atoms of hydrogen in CH_{4}. The scope of this work does not
permit of a more detailed account of this matter. It is given in
works on organic chemistry.
[44] The union of carbon atoms in closed chains or rings was first
suggested by Kekulé as an explanation of the structure and
isomerism of the derivatives of benzene, C_{6}H_{6}, forming
aromatic compounds (Note 26).
[45] The following are the most generally known of the oxygenised but
non-nitrogenous hydrocarbon derivatives. (1) the alcohols. These
are hydrocarbons in which hydrogen is exchanged for hydroxyl
(OH). The simplest of these is methyl alcohol, CH_{3}(OH),
or wood spirit obtained by the dry distillation of wood. The
common spirits of wine or ethyl alcohol, C_{2}H_{3}(OH), and
glycol, C_{2}H_{4}(OH)_{2}, correspond with ethane. Normal
propyl alcohol, CH_{3}CH_{2}CH_{2}(OH), and isopropyl alcohol,
CH_{3}CH(OH)CH_{3}, propylene-glycol, C_{3}H_{6}(OH)_{2}, and
glycerol, C_{3}H_{3}(OH)_{3} (which, with stearic and other
acids, forms fatty substances), correspond with propane,
C_{3}H_{8}. All alcohols are capable of forming water and
ethereal salts with acids, just as alkalis form ordinary salts.
(2) Aldehydes are alcohols minus hydrogen; for instance,
acetaldehyde, C_{2}H_{4}O, corresponds with ethyl alcohol.
(3) It is simplest to regard organic acids as hydrocarbons in
which hydrogen has been exchanged for carboxyl (CO_{2}H), as
will be explained in the following chapter. There are a number
of intermediate compounds; for example, the aldehyde-alcohols,
alcohol-acids (or hydroxy-acids), &c. Thus the hydroxy-acids are
hydrocarbons in which some of the hydrogen has been replaced
by hydroxyl, and some by carboxyl; for instance, lactic
acid corresponds with C_{2}H_{6}, and has the constitution
C_{2}H_{4}(OH)(CO_{2}H). If to these products we add the haloid
salts (where H is replaced by Cl, Br, I), the nitro-compounds
containing NO_{2} in place of H, the amides, cyanides, ketones,
and other compounds, it will be readily seen what an immense
number of organic compounds there are and what a variety of
properties these substances have; this we see also from the
composition of plants and animals.
Olefiant gas, almost free from other gases,[46] may be obtained from
ordinary alcohol (if possible, free from water) if it be mixed with
five parts of strong sulphuric acid and the mixture heated to slightly
above 100°. Under these conditions, the sulphuric acid removes the
elements of water from the alcohol, C_{2}H_{5}(OH), and gives olefiant
gas; C_{2}H_{6}O = H_{2}O + C_{2}H_{4}. The greater molecular weight
of olefiant gas compared with marsh gas indicates that it may be
comparatively easily converted into a liquid by means of pressure or
great cold; this may be effected, for example, by the evaporation
of liquid nitrous oxide. Its absolute boiling point is +10°, it
boils at -103° (1 atmosphere), liquefies at 0°, at a pressure of 43
atmospheres, and solidifies at -160°. Ethylene is colourless, has a
slight ethereal smell, is slightly soluble in water, and somewhat more
soluble in alcohol and in ether (in five volumes of spirit and six
volumes of ether).[47]
[46] Ethylene bromide, C_{2}H_{4}Br_{2}, when gently heated in
alcoholic solution with finely divided zinc, yields pure
ethylene, the zinc merely taking up the bromine (Sabaneyeff).
[47] Ethylene decomposes somewhat easily under the influence of the
electric spark, or a high temperature. In this case the volume
of the gas formed may remain the same when olefiant gas is
decomposed into carbon and marsh gas, or may increase to double
its volume when hydrogen and carbon are formed, C_{2}H_{4} =
CH_{4} + C = 2C + 2H_{2}. A mixture of olefiant gas and oxygen is
highly explosive; two volumes of this gas require six volumes of
oxygen for its perfect combustion. The eight volumes thus taken
then resolve themselves into eight volumes of the products of
combustion, a mixture of water and carbonic anhydride, C_{2}H_{4}
+ 3O_{2} = 2CO_{2} + 2H_{2}O. On cooling after the explosion
diminution of volume occurs because the water becomes liquid. For
two volumes of the olefiant gas taken, the diminution will be
equal to four volumes, and the same for marsh gas. The quantity
of carbonic anhydride formed by both gases is not the same. Two
volumes of marsh gas give only two volumes of carbonic anhydride,
and two volumes of ethylene give four volumes of carbonic
anhydride.
Like other unsaturated hydrocarbons, olefiant gas readily enters into
combination with certain substances, such as chlorine, bromine, iodine,
fuming sulphuric acid, or sulphuric anhydride, &c. If olefiant gas be
sealed up with a small quantity of sulphuric acid in a glass vessel, and
constantly agitated (as, for instance, by attaching it to the moving
part of a machine), the prolonged contact and repeated mixing causes
the olefiant gas, little by little, to combine with the sulphuric acid,
forming C_{2}H_{4}H_{2}SO_{4}. If, after this absorption, the sulphuric
acid be diluted with water and distilled, alcohol separates, which is
produced in this case by the olefiant gas combining with the elements of
water, C_{2}H_{4} + H_{2}O = C_{2}H_{6}O. In this reaction (Berthelot)
we see an excellent example of the fact that if a given substance, like
olefiant gas, is produced by the decomposition of another, then in the
reverse way this substance, entering into combination, is capable of
forming the original substance--in our example, alcohol. In combination
with various molecules, X_{2}, ethylene gives saturated compounds,
C_{2}H_{4}X_{2} or CH_{2}XCH_{2}X (for example, C_{2}H_{4}Cl_{2}), which
correspond with ethane, CH_{3}CH_{3} or C_{2}H_{6}.[48]
[48] The homologues of ethylene, C_{_n_}H_{2_n_}, are also capable
of direct combination with halogens, &c., but with various
degrees of facility. The composition of these homologues can be
expressed thus: (CH_{3})__x_(CH_{2})_{_y_}(CH)_{_z_}C_{_r_},
where the sum of _x_ + _z_ is always an even number, and
the sum of _x_ + _z_ + _r_ is equal to half the sum of
3_x_ + _z_, whence _z_ + 2_r_ = _x_; by this means the possible
isomerides are determined. For example, for butylenes,
C_{4}H_{8}, (CH_{3})_{2}(CH)_{2}, (CH_{3})_{2}(CH_{2})C,
(CH_{2})(CH_{2})_{2}CH, and (CH_{2})_{4} are possible.
_Acetylene_, C_{2}H_{2} = CHCH, is a gas; it was first prepared by
Berthelot (1857). It has a very pungent smell, is characterised by
its great stability under the action of heat, and is obtained as the
only product of the direct combination of carbon with hydrogen when a
luminous arc (voltaic) is formed between carbon electrodes. This arc
contains particles of carbon passing from one pole to the other. If the
carbons be surrounded with an atmosphere of hydrogen, the carbon in part
combines with the hydrogen, forming C_{2}H_{2}.[48 bis] Acetylene may be
formed from olefiant gas if two atoms of hydrogen be taken from it. This
may be effected in the following way: the olefiant gas is first made to
combine with bromine, giving C_{2}H_{4}Br_{2}; from this the hydrobromic
acid is removed by means of an alcoholic solution of caustic potash,
leaving the volatile product C_{2}H_{3}Br; and from this yet another
part of hydrobromic acid is withdrawn by passing it through anhydrous
alcohol in which metallic sodium has been dissolved, or by heating
it with a strong alcoholic solution of caustic potash. Under these
circumstances (Berthelot, Sawitsch, Miasnikoff) the alkali takes up the
hydrobromic acid from C_{_n_}H_{2_n_-1}Br, forming C_{_n_}H_{2_n_-2}.
[48 bis] _See_ also method of preparing C_{2}H_{2} in Note 12 bis.
Acetylene is also produced in all those cases where organic substances
are decomposed by the action of a high temperature--for example, by dry
distillation. On this account a certain quantity is always found in coal
gas, and gives to it, at all events in part, its peculiar smell, but
the quantity of acetylene in coal gas is very small. If the vapour of
alcohol be passed through a heated tube a certain quantity of acetylene
is formed. It is also produced by the imperfect combustion of olefiant
and marsh gas--for example, if the flame of coal gas has not free access
to air.[49] The inner part of every flame contains gases in imperfect
combustion, and in them some amount of acetylene.
[49] This is easily accomplished with those gas burners which are used
in laboratories and mentioned in the Introduction. In these
burners the gas is first mixed with air in a long tube, above
which it is kindled. But if it be lighted inside the pipe it
does not burn completely, but forms acetylene, on account of
the cooling effect of the walls of the metallic tube; this is
detected by the smell, and may be shown by passing the issuing
gas (by aid of an aspirator) into an ammoniacal solution of
cuprous chloride.
Acetylene, being further removed than ethylene from the limit
C_{_n_}H_{2_n_+2} of hydrocarbon compounds, has a still greater
faculty of combination than is shown by olefiant gas, and therefore can
be more readily separated from any mixture containing it. Actually,
acetylene not only combines with one and two molecules of I_{2}, HI,
H_{2}SO_{4}, Cl_{2}, Br_{2}, &c.... (many other unsaturated hydrocarbons
combine with them), but also with cuprous chloride, CuCl, forming a red
precipitate. If a gaseous mixture containing acetylene be passed through
an ammoniacal solution of cuprous chloride (or silver nitrate), the
other gases do not combine, but the acetylene gives a red precipitate
(or grey with silver), which detonates when struck with a hammer. This
red precipitate gives off acetylene under the action of acids. In this
manner pure acetylene may be obtained. Acetylene and its homologues also
readily react with corrosive sublimate, HgCl_{2} (Koucheroff, Favorsky).
Acetylene burns with a very brilliant flame, which is accounted for by
the comparatively large amount of carbon it contains.[50]
[50] Amongst the homologues of acetylene C_{_n_}H_{2_n_-2}, the lowest
is C_{3}H_{4}; allylene, CH_{3}CCH, and allene, CH_{2}CCH_{2},
are known, but the closed structure, CH_{2}(CH)_{2}, is little
investigated.
The formation and existence in nature of large masses of petroleum or a
mixture of liquid hydrocarbons, principally of the series C_{_n_}H_{2_n_
+2} and C_{_n_}H_{2_n_} is in many respects remarkable.[51] In some
mountainous districts--as, for instance, by the slopes of the Caucasian
chain, on inclines lying in a direction parallel to the range--an oily
liquid issues from the earth together with salt water and hot gases
(methane and others); it has a tarry smell and dark brown colour,
and is lighter than water. This liquid is called naphtha or rock oil
(petroleum) and is obtained in large quantities by sinking wells and
deep bore-holes in those places where traces of naphtha are observed,
the naphtha being sometimes thrown up from the wells in fountains of
considerable height.[52] The evolution of naphtha is always accompanied
by salt water and marsh gas. Naphtha has from ancient times been
worked in Russia in the Apsheron peninsula near Baku, and is also
now worked in Burmah (India), in Galicia near the Carpathians, and
in America, especially in Pennsylvania and Canada, &c. Naphtha does
not consist of one definite hydrocarbon, but of a mixture of several,
and its density, external appearance, and other qualities vary with
the amount of the different hydrocarbons of which it is composed.
The light kinds of naphtha have a specific gravity about 0·8 and the
heavy kinds up to 0·98. The former are very mobile liquids, and more
volatile; the latter contain less of the volatile hydrocarbons and are
less mobile. When the light kinds of naphtha are distilled, the boiling
point taken in the vapours constantly changes, beginning at 0° and
going up to above 350°. That which passes over first is a very mobile,
colourless ethereal liquid (forming gazolene, ligroin, benzoline,
&c.), from which the hydrocarbons whose boiling points start from 0°
may be extracted--namely, the hydrocarbons C_{4}H_{10}, C_{5}H_{12}
(which boils at 30°), C_{6}H_{14} (boils at 62°), C_{7}H_{16} (boils
about 90°), &c. Those fractions of the naphtha distillate which boil
above 130°, and contain hydrocarbons with C_{9}, C_{10}, C_{11}, &c.,
enter into the composition of the oily substance, universally used
for lighting, called kerosene or photogen or photonaphthalene, and by
other names. The specific gravity of kerosene is from 0·78 to 0·84, and
it smells like naphtha. Those products of the distillation of naphtha
which pass off below 130° and have a specific gravity below 0·75,
enter into the composition of light petroleum (benzoline, ligroin,
petroleum spirit, &c.); which is used as a solvent for india-rubber,
for removing grease spots, &c. Those portions of naphtha (which can
only be distilled without change by means of superheated steam,
otherwise they are largely decomposed) which boil above 275° and up to
300° and have a specific gravity higher than 0·85, form an excellent
oil,[53] safe as regards inflammability (which is very important
as diminishing the risks of fire), and may be used in lamps as an
effective substitute for kerosene.[54] Those portions of naphtha which
pass over at a still higher temperature and have a higher specific
gravity than 0·9, which are found in abundance (about 30 p.c.) in the
Baku naphtha, make excellent lubricating or machine oils. Naphtha has
many important applications, and the naphtha industry is now of great
commercial importance, especially as naphtha and its refuse may be
used as fuel.[55] Whether naphtha was formed from organic matter is
very doubtful, as it is found in the most ancient Silurian strata
which correspond with epochs of the earth's existence when there was
little organic matter; it could not penetrate from the higher to the
lower (more ancient) strata as it floats on water (and water penetrates
through all strata). It therefore tends to rise to the surface of the
earth, and it is always found in highlands parallel to the direction of
the mountains.[56] Much more probably its formation may be attributed
to the action of water penetrating through the crevasses formed on
the mountain slopes and reaching to the heart of the earth, to that
kernel of heated metallic matter which must be accepted as existing in
the interior of the earth. And as meteoric iron often contains carbon
(like cast iron), so, accepting the existence of such carburetted
iron at unattainable depths in the interior of the earth, it may be
supposed that naphtha was produced by the action of water penetrating
through the crevices of the strata during the upheaval of mountain
chains,[57] because water with iron carbide ought to give iron oxide
and hydrocarbons.[58] Direct experiment proves that the so-called
_spiegeleisen_ (manganiferous iron, rich in chemically combined
carbon) when treated with acids gives liquid hydrocarbons[59] which in
composition, appearance, and properties are completely identical with
naphtha.[60]
[51] The saturated hydrocarbons predominate in American petroleum,
especially in its more volatile parts; in Baku naphtha the
hydrocarbons of the composition C_{_n_}H_{2_n_} form the
main part (Lisenko, Markovnikoff, Beilstein) but doubtless
(Mendeléeff) it also contains saturated ones, C_{_n_}H_{2_n_+2}.
The structure of the naphtha hydrocarbons is only known for
the lower homologues, but doubtless the distinction between the
hydrocarbons of the Pennsylvanian and Baku naphthas, boiling at
the same temperature (after the requisite refining by repeated
fractional distillation, which can be very conveniently done
by means of steam rectification--that is, by passing the steam
through the dense mass), depends not only on the predominance
of saturated hydrocarbons in the former, and naphthenes,
C_{_n_}H_{2_n_}, in the latter, but also on the diversity of
composition and structure of the corresponding portions of the
distillation. The products of the Baku naphtha are richer in
carbon (therefore in a suitably constructed lamp they ought to
give a brighter light), they are of greater specific gravity, and
have greater internal friction (and are therefore more suitable
for lubricating machinery) than the American products collected
at the same temperature.
[52] The formation of naphtha fountains (which burst forth after the
higher clay strata covering the layers of sands impregnated
with naphtha have been bored through) is without doubt caused
by the pressure or tension of the combustible hydrocarbon
gases which accompany the naphtha, and are soluble in it under
pressure. Sometimes these naphtha fountains reach a height of 100
metres--for instance, the fountain of 1887 near Baku. Naphtha
fountains generally act periodically and their force diminishes
with the lapse of time, which might be expected, because the
gases which cause the fountains find an outlet, as the naphtha
issuing from the bore-hole carries away the sand which was
partially choking it up.
[53] This is a so-called intermediate oil (between kerosene and
lubricating oils), solar oil, or pyronaphtha. Lamps are already
being manufactured for burning it but still require improvement.
Above all, however, it requires a more extended market, and
this at present is wanting, owing to the two following reasons:
(1) Those products of the American petroleum which are the
most widely spread and almost universally consumed contain but
little of this intermediate oil, and what there is is divided
between the kerosene and the lubricating oils; (2) the Baku
naphtha, which is capable of yielding a great deal (up to 30
p.c.) of intermediate oil, is produced in enormous quantities,
about 300 million poods, but has no regular markets abroad,
and for the consumption in Russia (about 25 million poods of
kerosene per annum) and for the limited export (60 million
poods per annum) into Western Europe (by the Trans-Caucasian
Railway) those volatile and more dangerous parts of the naphtha
which enter into the composition of the American petroleum are
sufficient, although Baku naphtha yields about 25 p.c. of such
kerosene. For this reason pyronaphtha is not manufactured in
sufficient quantities, and the whole world is consuming the
unsafe kerosene. When a pipe line has been laid from Baku to the
Black Sea (in America there are many which carry the raw naphtha
to the sea-shore, where it is made into kerosene and other
products) then the whole mass of the Baku naphtha will furnish
safe illuminating oils, which without doubt will find an immense
application. A mixture of the intermediate oil with kerosene
or Baku oil (specific gravity 0·84 to 0·85) may be considered
(on removing the benzoline) to be the best illuminating oil,
because it is safe (flashing point from 40° to 60°), cheaper
(Baku naphtha gives as much as 60 p.c. of Baku oil), and burns
perfectly well in lamps differing but little from those made for
burning American kerosene (unsafe, flashing point 20° to 30°).
[54] The substitution of Baku pyronaphtha, or intermediate oil, or Baku
oil (_see_ Note 53), would not only be a great advantage as
regards safety from fire, but would also be highly economical.
A ton (62 poods) of American crude petroleum costs at the coast
considerably more than 24_s._ (12 roubles), and yields two-thirds
of a ton of kerosene suitable for ordinary lamps. A ton of raw
naphtha in Baku costs less than 4_s._ (1 rouble 80 copecks), and
with a pipe line to the shore of the Black Sea would not cost
more than 8 roubles, or 16_s._ Moreover, a ton of Baku naphtha
will yield as much as two-thirds of a ton of kerosene, Baku oil,
and pyronaphtha suitable for illuminating purposes.
[55] Naphtha has been applied for heating purposes on a large scale
in Russia, not only on account of the low cost of naphtha itself
and of the residue from the preparation of kerosene, but also
because the products of all the Baku naphtha do not find an
outlet for general consumption. Naphtha itself and its various
residues form excellent fuel, burning without smoke and giving
a high temperature (steel and iron may be easily melted in the
flame). A hundred poods of good coal (for instance, Don coal)
used as fuel for heating boilers are equivalent to 36 cubic feet
(about 250 poods) of dry wood, while only 70 poods of naphtha
will be required; and moreover there is no need for stoking, as
the liquid can be readily and evenly supplied in the required
quantity. The economic and other questions relating to American
and Baku petroleums have been discussed more in detail in some
separate works of mine (D. Mendeléeff): (1) 'The Naphtha Industry
of Pennsylvania and the Caucasus,' 1870; (2) 'Where to Build
Naphtha Works,' 1880; (3) 'On the Naphtha Question,' 1883; (4)
'The Baku Naphtha Question,' 1886; (5) the article on the naphtha
industry in the account of the Russian industries printed for the
Chicago Exhibition.
[56] As during the process of the dry distillation of wood, sea-weed,
and similar vegetable _débris_, and also when fats are decomposed
by the action of heat (in closed vessels), hydrocarbons similar
to those of naphtha are formed, it was natural that this fact
should have been turned to account to explain the formation
of the latter. But the hypothesis of the formation of naphtha
from vegetable _débris_ inevitably assumes coal to be the chief
element of decomposition, and naphtha is met with in Pennsylvania
and Canada, in the Silurian and Devonian strata, which do not
contain coal, and correspond to an epoch not abounding in organic
matter. Coal was formed from the vegetable _débris_ of the
Carboniferous, Jurassic, and other recent strata, but judging
more from its composition and structure, it has been subjected
to the same kind of decomposition as peat; nor could liquid
hydrocarbons have been thus formed to such an extent as we see
in naphtha. If we ascribe the derivation of naphtha to the
decomposition of fat (adipose, animal fat) we encounter three
almost insuperable difficulties: (1) Animal remains would furnish
a great deal of nitrogenous matter, whilst there is but very
little in naphtha; (2) the enormous quantity of naphtha already
discovered as compared with the insignificant amount of fat in
the animal carcase; (3) the sources of naphtha always running
parallel to mountain chains is completely inexplicable. Being
struck with this last-mentioned circumstance in Pennsylvania,
and finding that the sources in the Caucasus surround the whole
Caucasian range (Baku, Tiflis, Gouria, Kouban, Tamman, Groznoe,
Dagestan), I developed in 1876 the hypothesis of the mineral
origin of naphtha expounded further on.
[57] During the upheaval of mountain ranges crevasses would be formed
at the peaks with openings upwards, and at the foot of the
mountains with openings downwards. These cracks in course of time
fill up, but the younger the mountains the fresher the cracks
(the Alleghany mountains are, without doubt, more ancient than
the Caucasian, which were formed during the tertiary epoch);
through them water must gain access deep into the recesses of the
earth to an extent that could not occur on the level (on plains).
The situation of naphtha at the foot of mountain chains is the
principal argument in my hypothesis.
Another fundamental reason is the consideration of the mean
density of the earth. Cavendish, Airy, Cornu, Boys, and many
others who have investigated the subject by various methods,
found that, taking water = 1, the mean density of the earth
is nearly 5·5. As at the surface water and all rocks (sand,
clay, limestone, granite, &c.) have a density less than 3, it
is evident (as solid substances are but slightly compressible
even under the greatest pressure) that inside the earth there
are substances of a greater density--indeed, not less than 7
or 8. What conclusion, then, can be arrived at? Anything heavy
contained in the bosom of the earth must be distributed not
only on its surface, but throughout the whole solar system, for
everything tends to show that the sun and planets are formed from
the same material, and according to the hypothesis of Laplace
and Kant it is most probable, and indeed must necessarily be
held, that the earth and planets are but fragments of the solar
atmosphere, which have had time to cool considerably and become
masses semi-liquid inside and solid outside, forming both planets
and satellites. The sun amongst other heavy elements contains
a great deal of iron, as shown by spectrum analysis. There is
also much of it in an oxidised condition on the surface of the
earth. Meteoric stones, carried as fragmentary planets in the
solar system and sometimes falling upon the earth, consisting of
siliceous rocks similar to terrestrial ones, often contain either
dense masses of iron (for example, the Pallosovo iron preserved
in the St. Petersburg Academy of Sciences) or granular masses
(for instance, the Okhansk meteorite of 1886). It is therefore
possible that the interior of the earth contains much iron in a
metallic state. This might be anticipated from the hypothesis of
Laplace, for the iron must have been compressed into a liquid
at that period when the other component parts of the earth
were still strongly heated, and oxides of iron could not then
have been formed. The iron was covered with slags (mixtures of
silicates like glass fused with rocky matter) which did not allow
it to burn at the expense of the oxygen of the atmosphere or
of water, just at that time when the temperature of the earth
was very high. Carbon was in the same state; its oxides were
also capable of dissociation (Deville); it is also but slightly
volatile, and has an affinity for iron, and iron carbide is found
in meteoric stones (as well as carbon and even the diamond). Thus
the supposition of the existence of iron carbides in the interior
of the earth was derived by me from many indications, which
are to some extent confirmed by the fact that granular pieces
of iron have been found in some basalts (ancient lava) as well
as in meteoric stones. The occurrence of iron in contact with
carbon during the formation of the earth is all the more probable
because those elements predominate in nature which have small
atomic weights, and among them the most widely diffused, the most
difficultly fusible, and therefore the most easily condensed
(Chapter XV.) are carbon and iron. They passed into the liquid
state when all compounds were at a temperature of dissociation.
[58] The following is the typical equation for this formation:
3Fe_{_m_}C_{_n_} + 4_{_m_}H_{2}O = _m_Fe_{3}O_{4} (magnetic
oxide) + C_{5_n_}H_{8_m_} (_see_ Chapter XVII., Note 38).
[59] Cloez investigated the hydrocarbons formed when cast-iron is
dissolved in hydrochloric acid, and found C_{_n_}H_{2_n_} and
others. I treated crystalline manganiferous cast-iron with the
same acid, and obtained a liquid mixture of hydrocarbons exactly
similar to natural naphtha in taste, smell, and reaction.
[60] Probably naphtha was produced during the upheaval of all mountain
chains, but only in some cases were the conditions favourable
to its being preserved underground. The water penetrating below
formed there a mixture of naphtha and watery vapours, and this
mixture issued through fissures to the cold parts of the earth's
crust. The naphtha vapours, on condensing, formed naphtha,
which, if there were no obstacles, appeared on the surface
of land and water. Here part of it soaked through formations
(possibly the bituminous slates, schists, dolomites, &c., were
thus formed), another part was carried away on the water,
became oxidised, evaporated, and was driven to the shores (the
Caucasian naphtha probably in this way, during the existence of
the Aralo-Caspian sea, was carried as far as the Sisran banks
of the Volga, where many strata are impregnated with naphtha
and products of its oxidation resembling asphalt and pitch); a
great part of it was burnt in one way or another--that is, gave
carbonic anhydride and water. If the mixture of vapours, water,
and naphtha formed inside the earth had no free outlet to the
surface, it nevertheless would find its way through fissures to
the superior and colder strata, and there become condensed. Some
of the formations (clays) which do not absorb naphtha were only
washed away by the warm water, and formed mud, which we also now
observe issuing from the earth in the form of mud volcanoes. The
neighbourhood of Baku and the whole of the Caucasus near the
naphtha districts are full of such volcanoes, which from time to
time are in a state of eruption. In old naphtha beds (such as
the Pennsylvanian) even these blow-holes are closed, and the mud
volcanoes have had time to be washed away. The naphtha and the
gaseous hydrocarbons formed with it under the pressure of the
overlying earth and water impregnated the layers of sand, which
are capable of absorbing a great quantity of such liquid, and
if above this there were strata impermeable to naphtha (dense,
clayey, damp strata) the naphtha would accumulate in them. It is
thus preserved from remote geological periods up to the present
day, compressed and dissolved under the pressure of the gases
which burst out in places forming naphtha fountains. If this
be granted, it may be thought that in the comparatively new
(geologically speaking) mountain chains, such as the Caucasian,
naphtha is even now being formed. Such a supposition may explain
the remarkable fact that, in Pennsylvania, localities where
naphtha had been rapidly worked for five years have become
exhausted, and it becomes necessary to constantly have recourse
to sinking new wells in fresh places. Thus, from the year 1859,
the workings were gradually transferred along a line running
parallel to the Alleghany mountains for a distance of more
than 200 miles, whilst in Baku the industry dates from time
immemorial (the Persians worked near the village of Ballaghana)
and up to the present time keeps to one and the same place.
The amounts of the Pennsylvanian and Baku annual outputs are
at present equal--namely, about 250 million poods (4 million
tons). It may be that the Baku beds, as being of more recent
geological formation, are not so exhausted by nature as those of
Pennsylvania, and perhaps in the neighbourhood of Baku naphtha
is still being formed, which is partially indicated by the
continued activity of the mud volcanoes. As many varieties of
naphtha contain in solution solid slightly volatile hydrocarbons
like paraffin and mineral wax, the production of ozocerite,
or mountain wax, is accounted for in conjunction with the
formation of naphtha. Ozocerite is found in Galicia, also in
the neighbourhood of Novorossisk, in the Caucasus, and on the
islands of the Caspian Sea (particularly in the Chileken and Holy
Islands); it is met with in large masses, and is used for the
production of paraffin and _ceresene_, for the manufacture of
candles, and similar purposes.
As the naphtha treasures of the Caucasus have hardly been
exploited (near Baku and near Kouban and Grosnyi), and as naphtha
finds numerous uses, the subject presents most interesting
features to chemists and geologists, and is worthy of the close
attention of practical men.
CHAPTER IX
COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN
[Illustration: FIG. 61.--Dumas and Stas' apparatus for determining
the composition of carbonic anhydride. Carbon, graphite, or a diamond
is placed in the tube E in the furnace, and heated in a stream of
oxygen displaced from the bottle by water flowing from A. The oxygen
is purified from carbonic anhydride and water in the tubes B, C, D.
Carbonic anhydride, together with a certain amount of carbon monoxide,
is formed in E. The latter is converted into carbonic acid by passing
the products of combustion through a tube F, containing cupric oxide
heated in a furnace. The cupric oxide oxidises this CO into CO_{2},
forming metallic copper. The potash bulbs H and tubes I, J, K retain
the carbonic anhydride. Thus, knowing the weight of carbon taken and
the weight of the resultant carbonic anhydride (by weighing H, I, J, K
before and after the experiment), the composition of carbonic anhydride
and the equivalent of carbon may be determined.]
Carbonic anhydride (or carbonic acid or carbon dioxide, CO_{2}) was
the first of all gases distinguished from atmospheric air. Paracelsus
and Van Helmont, in the sixteenth century, knew that on heating
limestone a particular gas separated, which is also formed during the
alcoholic fermentation of saccharine solutions (for instance, in the
manufacture of wine); they knew that it was identical with the gas
which is produced by the combustion of charcoal, and that in some
cases it is found in nature. In course of time it was found that this
gas is absorbed by alkali, forming a salt which, under the action of
acid, again yields this same gas. Priestley found that this gas exists
in air, and Lavoisier determined its formation during respiration,
combustion, putrefaction, and during the reduction of the oxides of
metals by charcoal; he determined its composition, and showed that
it only contains oxygen and carbon. Berzelius, Dumas with Stas, and
Roscoe, determined its composition, showing that it contains twelve
parts of carbon to thirty-two of oxygen. The composition by volume
of this gas is determined from the fact that during the combustion
of charcoal in oxygen, the volume remains unchanged; that is to say,
_carbonic anhydride occupies the same volume as the oxygen which it
contains_--that is, the atoms of the carbon are, so to speak, squeezed
in between the atoms of the oxygen. O_{2} occupies two volumes and is
a molecule of ordinary oxygen; CO_{2} likewise occupies two volumes,
and expresses the composition and molecular weight of the gas. Carbonic
anhydride exists _in nature_, both in a free state and in the most
varied compounds. In a free state it is always contained (Chapter V.) in
the air, and in solution is in all kinds of water. It is evolved from
volcanoes, from mountain fissures, and in some caves. The well-known
Dog grotto, near Agnano on the bay of Baiæ, near Naples, furnishes the
best known example of such an evolution. Similar sources of carbonic
anhydride are also found in other places. In France, for instance, there
is a well-known poisonous fountain in Auvergne. It is a round hole,
surrounded with luxurious vegetation and constantly evolving carbonic
anhydride. In the woods surrounding the Lacher See near the Rhine, in
the neighbourhood of extinct volcanoes, there is a depression constantly
filled with this same gas. The insects which fly to this place perish,
animals being unable to breathe this gas. The birds chasing the insects
also die, and this is turned to profit by the local peasantry. Many
mineral springs carry into the air enormous quantities of this gas.
Vichy in France, Sprüdel in Germany, and Narzan in Russia (in Kislovodsk
near Piatigorsk) are known for their carbonated gaseous waters. Much
of this gas is also evolved in mines, cellars, diggings, and wells.
People descending into such places are suffocated. The combustion,
putrefaction, and fermentation of organic substances give rise to
the formation of carbonic anhydride. It is also introduced into the
atmosphere during the respiration of animals at all times and during
the respiration of plants in darkness and also during their growth.
Very simple experiments prove the formation of carbonic anhydride
under these circumstances; thus, for example, if the air expelled from
the lungs be passed through a glass tube into a transparent solution
of lime (or baryta) in water a white precipitate will soon be formed
consisting of an insoluble compound of lime and carbonic anhydride.
By allowing the seeds of plants to grow under a bell jar, or in a
closed vessel, the formation of carbonic anhydride may be similarly
confirmed. By confining an animal, a mouse, for instance, under a bell
jar, the quantity of carbonic acid which it evolves may be exactly
determined, and it will he found to be many grams per day for a mouse.
Such experiments on the respiration of animals have been also made with
great exactitude with large animals, such as men, bulls, sheep, &c. By
means of enormous hermetically closed bell receivers and the analysis
of the gases evolved during respiration it was found that a man expels
about 900 grams (more than two pounds) of carbonic anhydride per diem,
and absorbs during this time 700 grams of oxygen.[1] It must be remarked
that the carbonic anhydride of the air constitutes the fundamental
food of plants (Chapters III., V., and VIII.) Carbonic anhydride in a
state of combination with a variety of other substances is perhaps even
more widely distributed in nature than in a free state. Some of these
substances are very stable and form a large portion of the earth's
crust. For instance, limestones, calcium carbonate, CaCO_{3}, were
formed as precipitates in the seas existing previously on the earth;
this is proved by their stratified structure and the number of remains
of sea animals which they frequently contain. Chalk, lithographic stone,
limestone, marls (a mixture of limestone and clay), and many other rocks
are examples of such sedimentary formations. Carbonates with various
other bases--such as, for instance, magnesia, ferrous oxide, zinc oxide,
&c.--are often found in nature. The shells of molluscs also have the
composition CaCO_{3} and many limestones were exclusively formed from
the shells of minute organisms. As carbonic anhydride (together with
water) is produced during the combustion of all organic compounds in
a stream of oxygen or by heating them with substances which readily
part with their oxygen--for instance, with copper oxide--this method
is employed for estimating the amount of carbon in organic compounds,
more especially as the CO_{2} can be easily collected and the amount of
carbon calculated from its weight. For this purpose a hard glass tube,
closed at one end, is filled with a mixture of the organic substance
(about 0·2 gram) and copper oxide. The open end of the tube is fitted
with a cork and tube containing calcium chloride for absorbing the water
formed by the oxidation of the substance. This tube is hermetically
connected (by a caoutchouc tube) with potash bulbs or other weighing
apparatus (Chapter V.) containing alkali destined to absorb the carbonic
anhydride. The increase in weight of this apparatus shows the amounts of
carbonic anhydride formed during the combustion of the given substance,
and the quantity of carbon may be determined from this, because three
parts of carbon give eleven parts of carbonic anhydride.
[Illustration: FIG.--62. Apparatus for the combustion of organic
substances by igniting them with oxide of copper.]
[1] The quantity of carbonic acid gas exhaled by a man during the
twenty-four hours is not evenly produced; during the night more
oxygen is taken in than during the day (by night, in twelve hours,
about 450 grams), and more carbonic anhydride is separated by day
than during night-time and repose; thus, of the 900 grams produced
during the twenty-four hours about 375 are given out during the
night and 525 by day. This depends on the formation of carbonic
anhydride during the work performed by the man in the day. Every
movement is the result of some change of matter, for force cannot
be self-created (in accordance with the law of the conservation of
energy). Proportionally to the amount of carbon consumed an amount
of energy is stored up in the organism and is consumed in the
various movements performed by animals. This is proved by the fact
that during work a man exhales 525 grams of carbonic anhydride in
twelve hours instead of 375, absorbing the same amount of oxygen
as before. After a working day a man exhales by night almost the
same amount of carbonic anhydride as after a day of rest, so that
during a total twenty-four hours a man exhales about 900 grams
of carbonic anhydride and absorbs about 980 grams of oxygen.
Therefore during work the change of matter increases. The carbon
expended on the work is obtained from the food; on this account
the food of animals ought certainly to contain carbonaceous
substances capable of dissolving under the action of the digestive
fluids, and of passing into the blood, or, in other words, capable
of being digested. Such food for man and all other animals is
formed of vegetable matter, or of parts of other animals. The
latter in every case obtain their carbonaceous matter from plants,
in which it is formed by the separation of the carbon from the
carbonic anhydride taken up during the day by the respiration of
the plants. The volume of the oxygen exhaled by plants is almost
equal to the volume of the carbonic anhydride absorbed; that is to
say, nearly all the oxygen entering into the plant in the form of
carbonic anhydride is liberated in a free state, whilst the carbon
from the carbonic anhydride remains in the plant. At the same time
the plant absorbs moisture by its leaves and roots. By a process
which is unknown to us, this absorbed moisture and the carbon
obtained from the carbonic anhydride enter into the composition of
the plants in the form of so-called carbohydrates, composing the
greater part of the vegetable tissues, starch and cellulose of the
composition C_{6}H_{10}O_{5} being representatives of them. They
may be considered like all carbohydrates as compounds of carbon
and water, 6C + 5H_{2}O. In this way a _circulation_ of the carbon
goes on in nature by means of vegetable and animal organisms, in
which changes the principal factor is the carbonic anhydride of
the air.
_For the preparation of carbonic anhydride_ in laboratories and often
in manufactories, various kinds of calcium carbonate are used, being
treated with some acid; it is, however, most usual to employ the
so-called muriatic acid--that is, an aqueous solution of hydrochloric
acid, HCl--because, in the first place, the substance formed, calcium
chloride. CaCl_{2}, is soluble in water and does not hinder the further
action of the acid on the calcium carbonate, and secondly because, as
we shall see further on, muriatic acid is a common product of chemical
works and one of the cheapest. For calcium carbonate, either limestone,
chalk, or marble is used.[2]
CaCO_{3} + 2HCl = CaCl_{2} + H_{2}O + CO_{2}.
The nature of the reaction in this case is the same as in the
decomposition of nitre by sulphuric acid; only in the latter case a
hydrate is formed, and in the former an anhydride of the acid, because
the hydrate, carbonic acid, H_{2}CO_{3}, is unstable and as soon as it
separates decomposes into water and its own anhydride. It is evident
from the explanation of the cause of the action of sulphuric acid
on nitre that not every acid can be employed for obtaining carbonic
anhydride; namely, those will not set it free which chemically are
but slightly energetic, or those which are insoluble in water, or are
themselves as volatile as carbonic anhydride.[3] But as many acids are
soluble in water and are less volatile than carbonic anhydride, the
latter is evolved by the action of most acids on its salts, and this
reaction takes place at ordinary temperatures.[4]
[2] Other acids may be used instead of hydrochloric; for instance,
acetic, or even sulphuric, although this latter is not suitable,
because it forms as a product insoluble calcium sulphate (gypsum)
which surrounds the untouched calcium carbonate, and thus prevents
a further evolution of gas. But if porous limestone--for instance,
chalk--be treated with sulphuric acid diluted with an equal volume
of water, the liquid is absorbed and acting on the mass of the
salt, the evolution of carbonic anhydride continues evenly for a
long time. Instead of calcium carbonate other carbonates may of
course be used; for instance, washing-soda, Na_{2}CO_{3}, which
is often chosen when it is required to produce a rapid stream of
carbonic anhydride (for example, for liquefying it). But natural
crystalline magnesium carbonate and similar salts are with
difficulty decomposed by hydrochloric and sulphuric acids. When
for manufacturing purposes--for instance, in precipitating lime in
sugar-works--a large quantity of carbonic acid gas is required,
it is generally obtained by burning charcoal, and the products of
combustion, rich in carbonic anhydride, are pumped into the liquid
containing the lime, and the carbonic anhydride is thus absorbed.
Another method is also practised, which consists in using the
carbonic anhydride separated during fermentation, or that
evolved from limekilns. During the fermentation of sweet-wort,
grape-juice, and other similar saccharine solutions, the glucose
C_{6}H_{12}O_{6} changes under the influence of the yeast
organism, forming alcohol (2C_{2}H_{6}O), and carbonic anhydride
(2CO_{2}) which separates in the form of gas; if the fermentation
proceeds in closed bottles sparkling wine is obtained. When
carbonic acid gas is prepared for saturating water and other
beverages it is necessary to use it in a pure state. Whilst in
the state in which it is evolved from ordinary limestones by the
aid of acids it contains, besides a certain quantity of acid,
the organic matters of the limestone; in order to diminish the
quantity of these substances the densest kinds of dolomites are
used, which contain less organic matter, and the gas formed is
passed through various washing apparatus, and then through a
solution of potassium permanganate, which absorbs organic matter
and does not take up carbonic anhydride.
[3] Hypochlorous acid, HClO, and its anhydride, Cl_{2}O, do not displace
carbonic acid, and hydrogen sulphide has the same relation to
carbonic acid as nitric acid to hydrochloric--an excess of either
one displaces the other.
[4] Thus, in preparing the ordinary effervescing powders, sodium
bicarbonate (or acid carbonate of soda) is used, and mixed
with powdered citric or tartaric acid. In a dry state these
powders do not evolve carbonic anhydride, but when mixed with
water the evolution takes place briskly, which is due to the
substances passing into solution. The salts of carbonic acid
may be recognised from the fact that they evolve carbonic acid
with a hissing noise when treated with acids. If vinegar, which
contains acetic acid, be poured upon limestone, marble, malachite
(containing copper carbonate), &c., carbonic anhydride is evolved
with a hissing noise. It is noteworthy that neither hydrochloric
acid, nor even sulphuric acid nor acetic acid, acts on limestone
except in presence of water. We shall refer to this later on.
For the preparation of carbonic anhydride in laboratories, marble is
generally used. It is placed in a Woulfe's bottle and treated with
hydrochloric acid in an apparatus similar to the one used for the
production of hydrogen. The gas evolved carries away through the tube
part of the volatile hydrochloric acid, and it is therefore necessary to
wash the gas by passing it through another Woulfe's bottle containing
water. If it be necessary to obtain dry carbonic anhydride, it must be
passed through chloride of calcium.[5]
[5] The direct observations made (1876) by Messrs. Bogouski and
Kayander lead to the conclusion that the quantity of carbonic
anhydride evolved by the action of acids on marble (as homogeneous
as possible) is directly proportional to the time of action, the
extent of surface, and the degree of concentration of the acid,
and inversely proportional to the molecular weight of the acid.
If the surface of a piece of Carrara marble be equal to one
decimetre, the time of action one minute, and one cubic decimetre
or litre contains one gram of hydrochloric acid, then about 0·02
gram of carbonic anhydride will be evolved. If the litre contains
_n_ grams of hydrochloric acid, then by experiment the amount
will be _n_ × 0·02 of carbonic anhydride. Therefore, if the
litre contains 36·5 (= HCl) grams, about 0·73 gram of carbonic
anhydride (about half a litre) would he evolved per minute. If
nitric acid or hydrobromic acid be used instead of hydrochloric,
then, with a combining proportion of the acid, the same quantity
of carbonic anhydride will be evolved; thus, if the litre contains
63 (= HNO_{3}) grams of nitric acid, or 81 (= HBr) grams of
hydrobromic acid, the quantity of carbonic anhydride evolved will
still be 0·73 gram. Spring, in 1890, made a series of similar
determinations.
Carbonic anhydride may also be prepared by heating many of the salts of
carbonic acid; for instance, by heating magnesium carbonate, MgCO_{3}
(_e.g._, in the form of dolomite), the separation is easily effected,
particularly in the presence of the vapours of water. The acid salts
of carbonic acid (for instance, NaHCO_{3}, see further on) readily and
abundantly give carbonic anhydride when heated.
Carbonic anhydride is colourless, has a slight smell and a faint acid
taste; its density in a gaseous state is twenty-two times as great
as that of hydrogen, because its molecular weight is forty-four.[6]
It is an example of those gaseous substances which have been long
ago transformed into all the three states. In order to obtain liquid
carbonic anhydride, the gas must be submitted to a pressure of
thirty-six atmospheres at 0°.[7] Its absolute boiling point = +32°.[8]
Liquid carbonic anhydride is colourless, does not mix with water, but
is soluble in alcohol, ether, and oils; at 0° its specific gravity is
0·83.[8 bis] The boiling point of this liquid lies at -80°--that is to
say, the pressure of carbonic acid gas at that temperature does not
exceed that of the atmosphere. At the ordinary temperature the liquid
remains as such for some time under ordinary pressure, on account of
its requiring a considerable amount of heat for its evaporation. If
the evaporation takes place rapidly, especially if the liquid issues
in a stream, such a decrease of temperature occurs that a part of the
carbonic anhydride is transformed into a solid snowy mass. Water,
mercury, and many other liquids freeze on coming into contact with
snow-like carbonic anhydride.[9] In this form carbonic anhydride may be
preserved for a long time in the open air, because it requires still
more heat to turn it into a gas than when in a liquid state.[9 bis]
[6] As carbonic anhydride is one and a half times heavier than air, it
diffuses with difficulty, and therefore does not easily mix with
air, but sinks in it. This may be shown in various ways; for
instance, the gas may be carefully poured from one vessel into
another containing air. If a lighted taper be plunged into the
vessel containing carbonic anhydride it is extinguished, and then,
after pouring the gas into the other cylinder, it will burn in the
former and be extinguished in the latter. If a certain quantity
of carbonic anhydride be poured into a vessel containing air, and
soap-bubbles be introduced, they will only sink as far as the
stratum where the atmosphere of carbonic anhydride commences, as
this latter is heavier than the soap-bubbles filled with air.
Naturally, after a certain lapse of time, the carbonic anhydride
will be diffused throughout the vessel, and form a uniform mixture
with the air, just as salt in water.
[7] This liquefaction was first observed by Faraday, who sealed up in
a tube a mixture of a carbonate and sulphuric acid. Afterwards
this method was very considerably improved by Thilorier and
Natterer, whose apparatus is given in Chapter VI. in describing
N_{2}O. It is, however, necessary to remark that the preparation
of liquid carbonic anhydride requires good liquefying apparatus,
constant cooling, and a rapid preparation of large masses of
carbonic anhydride.
[8] Carbonic anhydride, having the same molecular weight as nitrous
oxide, very much resembles it when in a liquid state.
[8 bis] When poured into a tube, which is then sealed up, liquefied
carbonic anhydride can be easily preserved, because a thick tube
easily supports the pressure (about 50 atmospheres) exerted by the
liquid at the ordinary temperature.
[9] When a fine stream of liquid carbonic anhydride is discharged into
a closed metallic vessel, about one-third of its mass solidifies
and the remainder evaporates. In employing solid carbonic
anhydride for making experiments at low temperatures, it is best
to use it mixed with ether, otherwise there will be few points
of contact. If a stream of air be blown through a mixture of
liquid carbonic anhydride and ether, the evaporation proceeds
rapidly, and great cold is obtained. At present in some special
manufactories (and for making artificial mineral waters)
carbonic anhydride is liquefied on the large scale, filled into
wrought-iron cylinders provided with a valve, and in this manner
it can be transported and preserved safely for a long time. It is
used, for instance, in breweries.
[9 bis] Solid carbonic anhydride, notwithstanding its very low
temperature, can be safely placed on the hand, because it
continually evolves gas which prevents its coming into actual
contact with the skin, but if a piece be squeezed between the
fingers, it produces a severe frost bite similar to a burn. If the
snow-like solid be mixed with ether, a semi-liquid mass is
obtained, which is employed for artificial refrigeration. This
mixture may be used for liquefying many other gases--such as
chlorine, nitrous oxide, hydrogen sulphide, and others. The
evaporation of such a mixture proceeds with far greater rapidity
under the receiver of an air-pump, and consequently the
refrigeration is more intense. By this means many gases may be
liquefied which resist other methods--namely, olefiant gas,
hydrochloric acid gas, and others. Liquid carbonic anhydride in
this case congeals in the tube into a glassy transparent mass.
Pictet availed himself of this method for liquefying many permanent
gases (_see_ Chapter II.)
Bleekrode, by compressing solid CO_{2} in a cylinder by means of a
piston, obtained a semi-transparent stick, which contained as much
as 1·3 and even 1·6 gram of CO_{2} per cubic centimetre. In this
form the CO_{2} slowly evaporated, and could be kept for a long
time.
The capacity which carbonic anhydride has of being liquefied stands in
connection with its _considerable solubility in water_, alcohol, and
other liquids. Its solubility in water has been already spoken of in the
first chapter. Carbonic anhydride is still more soluble in alcohol than
in water, namely at 0° one volume of alcohol dissolves 4·3 volumes of
this gas, and at 20° 2·9 volumes.
Aqueous solutions of carbonic anhydride, under a pressure of several
atmospheres, are now prepared artificially, because water saturated
with this gas promotes digestion and quenches thirst. For this purpose
the carbonic anhydride is pumped by means of a force-pump into a closed
vessel containing the liquid, and then bottled off, taking special means
to ensure rapid and air-tight corking. Various effervescing drinks and
artificially effervescing wines are thus prepared. The presence of
carbonic anhydride has an important significance in nature, because by
its means water acquires the property of decomposing and dissolving many
substances which are not acted on by pure water; for instance, calcium
phosphates and carbonates are soluble in water containing carbonic acid.
If the water in the interior of the earth is saturated with carbonic
acid under pressure, the quantity of calcium carbonate in solution may
reach three grams per litre, and on issuing at the surface, as the
carbonic anhydride escapes, the calcium carbonate will be deposited.[10]
Water charged with carbonic anhydride brings about the destruction of
many rocky formations by removing the lime, alkali, &c., from them. This
process has been going on and continues on an enormous scale. Rocks
contain silica and the oxides of various metals; amongst others, the
oxides of aluminium, calcium, and sodium. Water charged with carbonic
acid dissolves both the latter, transforming them into carbonates. The
waters of the ocean ought, as the evolution of the carbonic anhydride
proceeds, to precipitate salts of lime; these are actually found
everywhere on the surface of the ground in those places which previously
formed the bed of the ocean. The presence of carbonic anhydride in
solution in water is essential to the nourishment and growth of water
plants.
[10] If such water trickles through crevices and enters a cavern, the
evaporation will be slow, and therefore in those places from
which the water drips growths of calcium carbonate will be
formed, just like the icicles formed on the roof-gutters in
winter-time. Similar conical and cylindrical stony growths form
the so-called stalactites or pendants hanging from above and
stalagmites formed on the bottom of caves. Sometimes these two
kinds meet together, forming entire columns filling the cave.
Many of these caves are remarkable for their picturesqueness; for
instance, the cave of Antiparos, in the Grecian Archipelago. This
same cause also forms spongy masses of calcium carbonate in those
places where the springs come to the surface of the earth. It is
therefore very evident that a calcareous solution is sometimes
capable of penetrating plants and filling the whole of their mass
with calcium carbonate. This is one of the forms of petrified
plants. Calcium phosphate in solution in water containing
carbonic acid plays an important part in the nourishment of
plants, because all plants contain both lime and phosphoric acid.
Although carbonic anhydride is soluble in water, yet no definite hydrate
is formed;[11] nevertheless an idea of the composition of this hydrate
may be formed from that of the salts of carbonic acid, because a hydrate
is nothing but a salt in which the metal is replaced by hydrogen.
As carbonic anhydride forms salts of the composition K_{2}CO_{3},
Na_{2}CO_{3}, HNaCO_{3}, &c., therefore carbonic acid ought to have
the composition H_{2}CO_{3}--that is, it ought to contain CO_{2} +
H_{2}O. Whenever this substance is formed, it decomposes into its
component parts--that is, into water and carbonic anhydride. _The acid
properties_ of carbonic anhydride[11 bis] are demonstrated by its being
directly absorbed by alkaline solutions and forming salts with them. In
distinction from nitric, HNO_{3}, and similar monobasic acids which with
univalent metals (exchanging one atom for one atom of hydrogen) give
salts such as those of potassium, sodium, and silver containing only one
atom of the metal (NaNO_{3}, AgNO_{3}), and with bivalent[12] metals
(such as calcium, barium, lead) salts containing two acid groups--for
example, Ca(NO_{3})_{2}, Pb(NO_{3})_{2}--carbonic acid, H_{2}CO_{3},
_is bibasic_, that is contains two atoms of hydrogen in the hydrate or
two atoms of univalent metals in their salts: for example, Na_{2}CO_{3}
is washing soda, a normal salt; NaHCO_{3} is the bicarbonate, an
acid salt. Therefore, if M´ be a univalent metal, its carbonates in
general are the normal carbonate M´_{2}CO_{3} and the acid carbonate,
M´HCO_{3}; or if M´´ be a bivalent metal (replacing H_{2}) its normal
carbonate will be M´´CO_{3}; these metals do not usually form acid
salts, as we shall see further on. The bibasic character of carbonic
acid is akin to that of sulphuric acid, H_{2}SO_{4},[13] but the
latter, in distinction from the former, is an example of the energetic
or strong acids (such as nitric or hydrochloric), whilst in carbonic
acid we observe but feeble development of the acid properties; hence
carbonic acid must be considered _a weak acid_. This conception must,
however, be taken as only comparative, as up to this time there is no
definitely established rule for measuring the energy[14] of acids. The
feeble acid properties of carbonic acid may, however, be judged from
the joint evidence of many properties. With such energetic alkalis
as soda and potash, carbonic acid forms normal salts, soluble in
water, but having an alkaline reaction and in many cases themselves
acting as alkalis.[15] The acid salts of these alkalis, NaHCO_{3} and
KHCO_{3}, have a neutral reaction on litmus, although they, like acids,
contain hydrogen, which may be exchanged for metals. The acid salts
of such acids--as, for instance, of sulphuric acid, NaHSO_{4}--have a
clearly defined acid reaction, and therefore carbonic acid is unable
to neutralise the powerful basic properties of such alkalis as potash
or soda. Carbonic acid does not even combine at all with feeble bases,
such as alumina, Al_{2}O_{3}, and therefore if a strong solution of
sodium carbonate, Na_{2}CO_{3}, be added to a strong solution of
aluminium sulphate, Al_{2}(SO_{4})_{3}, although according to double
saline decompositions aluminium carbonate, Al_{2}(CO_{3})_{3}, ought
to be formed, the carbonic acid separates, for this salt splits up in
the presence of water into aluminium hydroxide and carbonic anhydride:
Al_{2}(CO_{3})_{3} + 3H_{2}O = Al_{2}(OH)_{6} + 3CO_{2}. Thus feeble
bases are unable to retain carbonic acid even at ordinary temperatures.
For the same reason, in the case of bases of medium energy, although
they form carbonates, the latter are comparatively easily decomposed by
heating, as is shown by the decomposition of copper carbonate, CuCO_{3}
(_see_ Introduction), and even of calcium carbonate, CaCO_{3}. Only
the normal (not the acid) salts of such powerful bases as potassium
and sodium are capable of standing a red heat without decomposition.
The acid salts--for instance, NaHCO_{3}--decompose even on heating
their solutions (2NaHCO_{3} = Na_{2}CO_{3} + H_{2}O + CO_{2}), evolving
carbonic anhydride. The amount of heat given out by the combination of
carbonic acid with bases also shows its feeble acid properties, being
considerably less than with energetic acids. Thus if a weak solution
of forty grams of sodium hydroxide be saturated (up to the formation
of a normal salt) with sulphuric or nitric acid or another powerful
acid, from thirteen to fifteen thousand calories are given out, but
with carbonic acid only about ten thousand calories.[16] The majority
of carbonates are insoluble in water, and therefore such solutions as
sodium, potassium, or ammonium carbonates form in solutions of most
other salts, MX or M´´X_{2}, insoluble precipitates of carbonates,
M_{2}CO_{3} or M´´CO_{3}. Thus a solution of barium chloride gives with
sodium carbonate a precipitate of barium carbonate, BaCO_{3}. For this
reason rocks, especially those of aqueous origin, very often contain
carbonates; for example, calcium, ferrous, or magnesium carbonates, &c.
[11] The crystallohydrate, CO_{2},8H_{2}O of Wroblewski (Chapter 1.,
Note 67), in the first place, is only formed under special
conditions; in the second place, its existence still requires
confirmation; and in the third place, it does not correspond with
that hydrate H_{2}CO_{3} which should occur, judging from the
composition of the salts.
[11 bis] It is easy to demonstrate the acid properties of carbonic
anhydride by taking a long tube, closed at one end, and filling
it with this gas; a test-tube is then filled with a solution of
an alkali (for instance, sodium hydroxide), which is then poured
into the long tube and the open end is corked. The solution is
then well shaken in the tube, and the corked end plunged into
water. If the cork be now withdrawn under water, the water will
fill the tube. The vacuum obtained by the absorption of the
carbonic anhydride by an alkali is so complete that even an
electric discharge will not pass through it. This method is often
applied to produce a vacuum.
[12] The reasons for distinguishing the uni-, bi-, tri-, and
quadrivalent metals will be explained hereafter on passing from
the univalent metals (Na, K, Li) to the bivalent (Mg. Ca, Ba),
Chapter XIV.
[13] Up to the year 1840, or thereabout, acids were not distinguished
by their basicity. Graham, while studying phosphoric acid,
H_{3}PO_{4}, and Liebig, while studying many organic acids,
distinguished mono-, bi-, and tribasic acids. Gerhardt and Laurent
generalised these relations, showing that this distinction extends
over many reactions (for instance, to the faculty of bibasic acids
of forming acid salts with alkalis, KHO or NaHO, or with alcohols,
RHO, &c.); but now, since a definite conception as to atoms and
molecules has been arrived at, _the basicity of an acid is
determined by the number of hydrogen atoms,_ contained in a
molecule of the acid, which can be exchanged for metals. If
carbonic acid forms acid salts, NaHCO_{3}, and normal salts,
Na_{2}CO_{3}, it is evident that the hydrate is H_{2}CO_{3}, a
bibasic acid. Otherwise it is at present impossible to account for
the composition of these salts. But when C = 6 and O = 8 were
taken, then the formula CO_{2} expressed the composition, but not
the molecular weight, of carbonic anhydride; and the composition
of the normal salt would be Na_{2}C_{2}O_{6} or NaCO_{3},
therefore carbonic acid might have been considered as a monobasic
acid. Then the acid salt would have been represented by
NaCO_{3},HCO_{3}. Such questions were the cause of much argument
and difference of opinion among chemists about forty years ago. At
present there cannot be two opinions on the subject if the law of
Avogadro-Gerhardt and its consequences be strictly adhered to. It
may, however, be observed here that the monobasic acids R(OH) were
for a long time considered to be incapable of being decomposed
into water and anhydride, and this property was ascribed to the
bibasic acids R(OH)_{2} as containing the elements necessary for
the separation of the molecule of water, H_{2}O. Thus H_{2}SO_{4}
or SO_{2}(OH)_{2}, H_{2}CO_{3}, or CO(OH)_{2}, and other bibasic
acids decompose into an anhydride, RO, and water, H_{2}O. But as
nitrous, HNO_{2}, iodic, HIO_{3}, hypochlorous, HClO, and other
monobasic acids easily give their anhydrides N_{2}O_{3},
I_{2}O_{5}, Cl_{2}O, &c., that method of distinguishing the
basicity of acids, although it fairly well satisfies the
requirements of organic chemistry, cannot be considered correct.
It may also be remarked that up to the present time not one of the
bibasic acids has been found to have the faculty of being
distilled without being decomposed into anhydride and water (even
H_{2}SO_{4}, on being evaporated and distilled, gives SO_{3} +
H_{2}O), and the decomposition of acids into water and anhydride
proceeds particularly easily in dealing with feebly energetic
acids, such as carbonic, nitrous, boric, and hypochlorous. Let us
add that carbonic acid, as a hydrate corresponding to marsh gas,
C(HO)_{4} = CO_{2} + 2H_{2}O, ought to be tetrabasic. But in
general it does not form such salts. Basic salts, however, such as
CuCO_{3}CuO, may be regarded in this sense, for CCu_{2}O_{4}
corresponds with CH_{4}O_{4}, as Cu corresponds with H_{2}.
Amongst the ethereal salts (alcoholic derivatives) of carbonic
acid corresponding cases are, however, observed; for instance,
ethylic orthocarbonate, C(C_{2}H_{5}O)_{4} (obtained by the action
of chloropicrin, C(NO_{2})Cl_{3}, on sodium ethoxide,
C_{2}H_{5}ONa; boiling point 158°; specific gravity, 0·92). The
name _orthocarbonic acid_ for CH_{4}O_{4} is taken from
_orthophosphoric acid_, PH_{3}O_{4}, which corresponds with PH_{3}
(_see_ Chapter on Phosphorus).
[14] Long ago endeavours were made to find a _measure of affinity_ of
acids and bases, because some of the acids, such as sulphuric or
nitric, form comparatively stable salts, decomposed with
difficulty by heat and water, whilst others, like carbonic and
hypochlorous acids, do not combine with feeble bases, and with
most of the other bases form salts which are easily decomposed.
The same may be said with regard to bases, among which those of
potassium, K_{2}O, sodium, Na_{2}O, and barium, BaO, may serve as
examples of the most powerful, because they combine with the most
feeble acids and form a mass of salts of great stability, whilst
as examples of the feeblest bases alumina, Al_{2}O_{3}, or bismuth
oxide, Bi_{2}O_{3}, may be taken, because they form salts easily
decomposed by water and by heat if the acid be volatile. Such a
division of acids and bases into the feeblest and most powerful is
justified by all evidence concerning them, and is quoted in this
work. But the teaching of this subject in certain circles has
acquired quite a new tone, which, in my opinion, cannot be
accepted without certain reservations and criticisms, although it
comprises many interesting features. The fact is that Thomsen,
Ostwald, and others proposed to express the measure of affinity of
acids to bases by figures drawn from data of the measure of
displacement of acids in aqueous solutions, judging (1) from the
amount of heat developed by mixing a solution of the salt with a
solution of another acid (the avidity of acids, according to
Thomsen); (2) from the change of the volumes accompanying such a
mutual action of solutions (Ostwald); (3) from the change of the
index of refraction of solutions (Ostwald), &c. Besides this there
are many other methods which allow us to form an opinion about the
distribution of bases among various acids in aqueous solutions.
Some of these methods will be described hereafter. It ought,
however, to be remarked that in making investigations in aqueous
solutions the affinity to water is generally left out of sight. If
a base N, combining with acids X and Y in presence of them both,
divides in such a way that one-third of it combines with X and
two-thirds with Y, a conclusion is formed that the affinity, or
power of forming salts, of the acid Y is twice as great as that of
X. But the presence of the water is not taken into account. If the
acid X has an affinity for water and for N it will be distributed
between them; and if X has a greater affinity for water than Y,
then less of X will combine with N than of Y. If, in addition to
this, the acid X is capable of forming an acid salt NX_{2}, and Y
is not, the conclusion of the relative strength of X and Y will be
still more erroneous, because the X set free will form such a salt
on the addition of Y to NX. We shall see in Chapter X. that when
sulphuric and nitric acids in weak aqueous solution act on sodium,
they are distributed exactly in this way: namely, one-third of the
sodium combines with the sulphuric and two-thirds with the nitric
acid; but, in my opinion, this does not show that sulphuric acid,
compared with nitric acid, possesses but half the degree of
affinity for bases like soda, and only demonstrates the greater
affinity of sulphuric acid for water compared with that of nitric
acid. In this way the methods of studying the distribution in
aqueous solutions probably only shows the difference of the
relation of the acid to a base and to water.
In view of these considerations, although the teaching of the
distribution of salt-forming elements in _aqueous solutions_ is an
object of great and independent interest, it can hardly serve to
determine the measure of affinity between bases and acids. Similar
considerations ought to be kept in view when determining the
energy of acids by means of the _electrical conductivity of their
weak solutions_. This method, proposed by Arrhenius (1884), and
applied on an extensive scale by Ostwald (who developed it in
great detail in his _Lehrbuch d. allgemeinen Chemie_, v. ii.,
1887), is founded on the fact that the relation of the so-called
molecular electrical-conductivity of weak solutions of various
acids (I) coincides with the relation in which the same acids
stand according to the distribution, (II) found by one of the
above-mentioned methods, and with the relation deduced for them
from observations upon the velocity of reaction, (III) for
instance, according to the rate of the splitting up of an ethereal
salt (into alcohol and acid), or from the rate of the so-called
inversion of sugar--that is, its transformation into glucose--as
is seen by comparing the annexed figures, in which the energy of
hydrochloric acid is taken as equal to 100:--
I II III
Hydrochloric acid, HCl 100 100 100
Hydrobromic acid, HBr 101 98 105
Nitric acid, HNO_{3} 100 100 96
Sulphuric acid, H_{2}SO_{4} 65 49 74
Formic acid, CH_{2}O_{2} 2 4 1
Acetic acid, C_{2}H_{4}O_{2} 1 2 1
Oxalic acid, C_{2}H_{2}O_{4} 20 24 18
Phosphoric acid, PH_{3}O_{4} 7 -- 6
The coincidence of these figures, obtained by so many various
methods, presents a most important and instructive relation
between phenomena of different kinds, but in my opinion it does
not permit us to assert that the degree of affinity existing
between bases and various acids is determined by all these various
methods, because the influence of the water must be taken into
consideration. On this account, until the theory of solution is
more thoroughly worked out, this subject (which for the present
ought to be treated of in special treatises on chemical mechanics)
must be treated with great caution. But now we may hope to decide
this question guided by a study of the rate of reaction, the
influence of acids and bases upon indicators, &c., all of which
are treated fully in works on physical and theoretical chemistry.
[15] Thus, for instance, in the washing of fabrics the caustic alkalis,
such as sodium hydroxide, in weak solutions, act in removing the
fatty matter just in the same way as carbonate solutions; for
instance, a solution of soda crystals, Na_{2}CO_{3}. Soap acts in
the same way, being composed of feeble acids, either fatty or
resinous, combined with alkali. On this account all such
substances are applied in manufacturing processes, and answer
equally well in practice for bleaching and washing fabrics. Soda
crystals or soap are preferred to caustic alkali, because an
excess of the latter may have a destructive effect on the fabrics.
It may be supposed that in aqueous solutions of soap or soda
crystals, part of the base will form caustic alkali; that is to
say, the water will compete with the weak acids, and the alkali
will be distributed between them and the water.
[16] Although carbonic acid is reckoned among the feeble acids, yet
there are evidently many others still feebler--for instance,
prussic acid, hypochlorous acid, many organic acids, &c. Bases
like alumina, or such feeble acids as silica, when in combination
with alkalis, are decomposed in aqueous solutions by carbonic
acid, but on fusion--that is, without the presence of water--they
displace it, which clearly shows in phenomena of this kind how
much depends upon the conditions of reaction and the properties of
the substances formed. These relations, which at first sight
appear complex, may be best understood if we represent that two
salts, MX and NY, in general always give more or less of two other
salts, MY and NX, and then examine the properties of the derived
substances. Thus, in solution, sodium silicate, Na_{2}SiO_{3},
with carbonic anhydride will to some extent form sodium carbonate
and silica, SiO_{2}; but the latter, being colloid, separates, and
the remaining mass of sodium silicate is again decomposed by
carbonic anhydride, so that finally silica separates and sodium
carbonate is formed. In a fused state the case is different;
sodium carbonate will react with silica to form carbonic anhydride
and sodium silicate, but the carbonic anhydride will be separated
as a gas, and therefore in the residue the same reaction will
again take place, and ultimately the carbonic anhydride is
entirely eliminated and sodium silicate remains. If, on the other
hand, nothing is removed from the sphere of the reaction,
distribution takes place. Therefore, although carbonic anhydride
is a feeble acid, still not for this reason, but only in virtue of
its gaseous form, do all soluble acids displace it in saline
solutions (_see_ Chapter X.)
Carbonic anhydride--which, like water, is formed with the development
of a large amount of heat--is very stable. Only very few substances
are capable of depriving it of its oxygen. However, certain metals,
such as magnesium, potassium and the like, on being heated, burn in
it, depositing carbon and forming oxides. If a mixture of carbonic
anhydride and hydrogen be passed through a heated tube, the formation
of water and carbonic oxide will be observed; CO_{2} + H_{2} = CO
+ H_{2}O. But only a portion of the carbonic acid gas undergoes
this change, and therefore the result will be a mixture of carbonic
anhydride, carbonic oxide, hydrogen, and water, which does not suffer
further change under the action of heat.[17] Although, like water,
carbonic anhydride is exceedingly stable, still on being heated it
partially decomposes into carbonic oxide and oxygen. Deville showed that
such is the case if carbonic anhydride be passed through a long tube
containing pieces of porcelain and heated to 1,300°. If the products
of decomposition--namely, the carbonic oxide and oxygen--be suddenly
cooled, they can be collected separately, although they partly reunite
together. A similar decomposition of carbonic anhydride into carbonic
oxide and oxygen takes place on passing a series of electric sparks
through it (for instance, in the eudiometer). Under these conditions
an increase of volume occurs, because two volumes of CO_{2} give two
volumes of CO and one volume of O. The decomposition reaches a certain
limit (less than one-third) and does not proceed further, so that the
result is a mixture of carbonic anhydride, carbonic oxide, and oxygen,
which is not altered in composition by the continued action of the
sparks. This is readily understood, as it is a reversible reaction.
If the carbonic anhydride be removed, then the mixture explodes when
a spark is passed and forms carbonic anhydride.[17 bis] If from an
identical mixture the oxygen (and not the carbonic anhydride) be
removed, and a series of sparks be again passed, the decomposition is
renewed, and terminates with the complete dissociation of the carbonic
anhydride. Phosphorus is used in order to effect the complete absorption
of the oxygen. In these examples we see that a definite mixture of
changeable substances is capable of arriving at a state of stable
equilibrium, destroyed, however, by the removal of one of the substances
composing the mixture. This is one of the instances of the influence of
mass.
[17] Hydrogen and carbon are near akin to oxygen as regards affinity,
but it ought to be considered that the affinity of hydrogen is
slightly greater than that of carbon, because during the
combustion of hydrocarbons the hydrogen burns first. Some idea of
this similarity of affinity may be formed by the quantity of heat
evolved. Gaseous hydrogen, H_{2}, on combining with an atom of
oxygen, O = 16, develops 69,000 heat-units if the water formed be
condensed to a liquid state. If the water remains in the form of a
gas (steam) the latent heat of evaporation must be subtracted, and
then 58,000 calories will be developed. Carbon, C, as a solid, on
combining with O_{2} = 32 develops about 97,000 calories, forming
gaseous CO_{2}. If it were gaseous like hydrogen, and only
contained C_{2} in its molecule, much more heat would be
developed, and judging by other substances, whose molecules on
passing from the solid to the gaseous state absorb about 10,000 to
15,000 calories, it must be held that gaseous carbon on forming
gaseous carbonic anhydride would develop not less than 110,000
calories--that is, approximately twice as much as is developed in
the formation of water. And since there is twice as much oxygen in
a molecule of carbonic anhydride as in a molecule of water, the
oxygen develops approximately the same quantity of heat on
combining with hydrogen as with carbon. That is to say, that here
we find the same close affinity (_see_ Chapter II., Note 7)
determined by the quantity of heat as between hydrogen, zinc, and
iron. For this reason here also, as in the case of hydrogen and
iron, we ought to expect an equal distribution of oxygen between
hydrogen and carbon, if they are both in excess compared with the
amount of oxygen; but if there be an excess of carbon, it will
decompose water, whilst an excess of hydrogen will decompose
carbonic anhydride. Even if these phenomena and similar ones have
been explained in isolated cases, a complete theory of the whole
subject is still wanting in the present condition of chemical
knowledge.
[17 bis] The degree or relative magnitude of the dissociation of CO_{2}
varies with the temperature and pressure--that is, it increases
with the temperature and as the pressure decreases. Deville found
that at a pressure of 1 atmosphere in the flame of carbonic oxide
burning in oxygen, about 40 per cent. of the CO_{2}, is decomposed
when the temperature is about 3,000°, and at 1,500° less than 1
per cent. (Krafts); whilst under a pressure of 10 atmospheres
about 34 per cent. is decomposed at 3,300° (Mallard and Le
Chatelier). It follows therefore that, under very small pressures,
the dissociation of CO_{2} will be considerable even at
comparatively moderate temperatures, but at the temperature of
ordinary furnaces (about 1,000°) even under the small partial
pressure of the carbonic acid, there are only small traces of
decomposition which may be neglected in a practical estimation of
the combustion of fuels. We may here cite the molecular specific
heat of CO_{2} (_i.e._ the amount of heat required to raise 44
units of weight of CO_{2} 1°), according to the determinations and
calculations of Mallard and Le Chatelier, for a constant volume
C_{v} = 6·26 + 0·0037_t_; for a constant pressure C_{p} = C_{v} +
2 (_see_ Chapter XIV., Note 7), _i.e._ the specific heat of CO_{2}
increases rapidly with a rise of temperature: for example, at 0°
(per 1 part by weight), it is, at a constant pressure = 0·188, at
1,000° = 0·272, at 2,000°, about 0·356. A perfectly distinct rise
of the specific heat (for example, at 2,000°, 0·409), is given by
a comparison of observations made by the above-mentioned
investigators and by Berthelot and Vieille (Kournakoff). The cause
of this must be looked for in dissociation. T. M. Cheltzoff,
however, considers upon the basis of his researches upon
explosives that it must be admitted that a maximum is reached at a
certain temperature (about 2,500°), beyond which the specific heat
begins to fall.
Although carbonic anhydride is decomposed on heating, yielding
oxygen, it is nevertheless, like water, an unchangeable substance at
ordinary temperatures. Its decomposition, as effected by plants, is on
this account all the more remarkable; in this case the whole of the
oxygen of the carbonic anhydride is separated in the free state. The
mechanism of this change is that the heat and light absorbed by the
plants are expended in the decomposition of the carbonic anhydride.
This accounts for the enormous influence of temperature and light on
the growth of plants. But it is at present not clearly understood how
this takes place, or by what separate intermediate reactions the whole
process of decomposition of carbonic anhydride in plants into oxygen and
the carbohydrates (Note 1) remaining in them, takes place. It is known
that sulphurous anhydride (in many ways resembling carbonic anhydride)
under the action of light (and also of heat) forms sulphur and sulphuric
anhydride, SO_{3}, and in the presence of water, sulphuric acid. But
no similar decomposition has been obtained directly with carbonic
anhydride, although it forms an exceedingly easily decomposable higher
oxide--percarbonic acid;[18] and perhaps that is the reason the oxygen
separates. On the other hand, it is known that plants always form and
contain _organic acids_, and these must be regarded as derivatives of
carbonic acid, as is seen by all their reactions, of which we will
shortly treat. For this reason it might be thought that the carbonic
acid absorbed by the plants first forms (according to Baeyer) formic
aldehyde, CH_{2}O, and from it organic acids, and that these latter
in their final transformation form all the other complex organic
substances of the plants. Many organic acids are found in plants in
considerable quantity; for instance, tartaric acid, C_{4}H_{6}O_{6},
found in grape-juice and in the acid juice of many plants; malic acid,
C_{4}H_{6}O_{5}, found not only in unripe apples but in still larger
quantities in mountain ash berries; citric acid, C_{6}H_{8}O_{7}, found
in the acid juice of lemons, in gooseberries, cranberries, &c.; oxalic
acid, C_{2}H_{2}O_{4}, found in wood-sorrel and many other plants.
Sometimes these acids exist in a free state in the plants, and sometimes
in the form of salts; for instance, tartaric acid is met with in grapes
as the salt known as cream of tartar, but in the impure state called
argol, or tartar, C_{4}H_{5}KO_{6}. In sorrel we find the so-called
salts of sorrel, or acid potassium oxalate, C_{2}HKO_{4}. There is a
very clear connection between carbonic anhydride and the above-mentioned
organic acids--namely, they all, under one condition or another, yield
carbonic anhydride, and can all be formed by means of it from substances
destitute of acid properties. The following examples afford the best
demonstration of this fact: if acetic acid, C_{2}H_{4}O_{2}, the acid
of vinegar, be passed in the form of vapour through a heated tube, it
splits up into carbonic anhydride and marsh gas = CO_{2} + CH_{4}. But
conversely it can also be obtained from those components into which it
decomposes. If one equivalent of hydrogen in marsh gas be replaced (by
indirect means) by sodium, and the compound CH_{3}Na is obtained, this
directly absorbs carbonic anhydride, forming a salt of acetic acid,
CH_{3}Na + CO_{2} = C_{2}H_{3}NaO_{2}; from this acetic acid itself
may be easily obtained. Thus acetic acid decomposes into marsh gas
and carbonic anhydride, and conversely is obtainable from them. The
hydrogen of marsh gas does not, like that in acids, show the property
of being directly replaced by metals; _i.e._ CH_{4} does not show any
acid character whatever, but on combining with the elements of carbonic
anhydride it acquires the properties of an acid. The investigation
of all other organic acids shows similarly that their acid character
depends on their containing the elements of carbonic anhydride. For this
reason there is no organic acid containing less oxygen in its molecule
than there is in carbonic anhydride; every organic acid contains in
its molecule at least two atoms of oxygen. In order to express the
relation between carbonic acid, H_{2}CO_{3}, and organic acids, and in
order to understand the reason of the acidity of these latter, it is
simplest to turn to that law of substitution which shows (Chapter VI.)
the relation between the hydrogen and oxygen compounds of nitrogen,
and permits us (Chapter VIII.) to regard all hydrocarbons as derived
from methane. If we have a given organic compound, A, which has not the
properties of an acid, but contains hydrogen connected to carbon, as in
hydrocarbons, then ACO_{2} will be a monobasic organic acid, A2CO_{2}
a bibasic, A3CO_{2} a tribasic, and so on--that is, each molecule of
CO_{2} transforms one atom of hydrogen into that state in which it may
be replaced by metals, as in acids. This furnishes a direct proof that
in organic acids it is necessary to recognise the group HCO_{2}, or
carboxyl. If the addition of CO_{2} raises the basicity, the removal of
CO_{2} lowers it. Thus from the bibasic oxalic acid, C_{2}H_{2}O_{4}, or
phthalic acid, C_{8}H_{6}O_{4}, by eliminating CO_{2} (easily effected
experimentally) we obtain the monobasic formic acid, CH_{2}O_{2}, or
benzoic acid, C_{7}H_{6}O_{2}, respectively. The nature of carboxyl is
directly explained by the law of substitution. Judging from what has
been stated in Chapters VI. and VIII. concerning this law, it is evident
that CO_{2} is CH_{4} with the exchange of H_{4} for O_{2}, and that
the hydrate of carbonic anhydride, H_{2}CO_{3}, is CO(OH)_{2}, that is,
methane, in which two parts of hydrogen are replaced by two parts of
the water radical (OH, hydroxyl) and the other two by oxygen. Therefore
the group CO(OH), or carboxyl, HCO_{2}, is a part of carbonic acid,
and is equivalent to (OH), and therefore also to H. That is, it is a
univalent residue of carbonic acid capable of replacing one atom of
hydrogen. Carbonic acid itself is a bibasic acid, both hydrogen atoms
in it being replaceable by metals, therefore carboxyl, which contains
one of the hydrogen atoms of carbonic acid, represents a group in which
the hydrogen is exchangeable for metals. And therefore if 1, 2 ... _n_
atoms of non-metallic hydrogen are exchanged 1, 2 ... _n_ times for
carboxyl, we ought to obtain 1, 2 ... _n_-basic acids. _Organic acids
are the products of the carboxyl substitution in hydrocarbons._[18 bis]
If in the saturated hydrocarbons, C_{n}H_{2n + 2}, one part of
hydrogen is replaced by carboxyl, the monobasic saturated (or fatty)
acids, C_{n}H_{2n + 1}(CO_{2}H), will be obtained, as, for instance,
formic acid, HCO_{2}H, acetic acid, CH_{2}CO_{2}H, ... stearic acid,
C_{17}H_{35}CO_{2}H, &c. The double substitution will give bibasic
acids, C_{n}H_{2n}(CO_{2}H)(CO_{2}H); for instance, oxalic acid _n_
= 0, malonic acid _n_ = 1, succinic acid _n_ = 2, &c. To benzene,
C_{6}H_{6} correspond benzoic acid, C_{6}H_{5}(CO_{2}H), phthalic acid
(and its isomerides), C_{6}H_{4}(CO_{2}H)_{2}, up to mellitic acid,
C_{6}(CO_{2}H)_{6}, in all of which the basicity is equal to the number
of carboxyl groups. As many isomerides exist in hydrocarbons, it is
readily understood not only that such can exist also in organic acids,
but that their number and structure may be foreseen. This complex and
most interesting branch of chemistry is treated separately in organic
chemistry.
[18] Percarbonic acid, H_{2}CO_{4} (= H_{2}CO_{3} + O) is supposed
by A. Bach (1893) to be formed from carbonic acid in the action of
light upon plants, (in the same manner as, according to the above
scheme, sulphuric acid from sulphurous) with the formation of
carbon, which remains in the form of hydrates of carbon:
3H_{2}CO_{3} = 2H_{2}CO_{4} + CH_{2}O. This substance CH_{2}O
expresses the composition of formic aldehyde which, according to
Baeyer, by polymerisation and further changes, gives other
hydrates of carbon and forms the first product which is formed in
plants from CO_{2}. And Berthelot (1872) had already, at the time
of the discovery of persulphuric (Chapter XX.) and pernitric
(Chapter VI., Note 26) acids pointed out the formation of the
unstable percarbonic anhydride, CO_{3}. Thus, notwithstanding the
hypothetical nature of the above equation, it may be admitted all
the more as it explains the comparative abundance of peroxide of
hydrogen (Schöne, Chapter IV.) in the air, and this also at the
period of the most energetic growth of plants (in July), because
percarbonic acid should like all peroxides easily give H_{2}O_{2}.
Besides which Bach (1894) showed that, in the first place, traces
of formic aldehyde and oxidising agents (CO_{3} or H_{2}O_{2})
are formed under the simultaneous action of CO_{2} and sunlight
upon a solution containing a salt of uranium (which is oxidised),
and diethylaniline (which reacts with CH_{2}O), and secondly, that
by subjecting BaO_{2}, shaken up in water, to the action of a
stream of CO_{2} in the cold, extracting (also in the cold) with
ether, and then adding an alcoholic solution of NaHO, crystalline
plates of a sodium salt may be obtained, which with water evolve
oxygen and leave sodium carbonate; they are therefore probably the
per-salt. All these facts are of great interest and deserve
further verification and elaboration.
[18 bis] If CO_{2} is the anhydride of a bibasic acid, and carboxyl
corresponds with it, replacing the hydrogen of hydrocarbons, and
giving them the character of comparatively feeble acids, then
SO_{3} is the anhydride of an energetic bibasic acid, and
_sulphoxyl_, SO_{2}(OH), corresponds with it, being capable of
replacing the hydrogen of hydrocarbons, and forming comparatively
energetic _sulphur oxyacids_ (_sulphonic acids_); for instance,
C_{6}H_{5}(COOH), benzoic acid, and C_{6}H_{5}(SO_{2}OH),
benzenesulphonic acid, are derived from C_{6}H_{6}. As the
exchange of H for methyl, CH_{3}, is equivalent to the addition of
CH_{2}, the exchange of carboxyl, COOH, is equivalent to the
addition of CO_{2}; so the exchange of H for sulphoxyl is
equivalent to the addition of SO_{3}. The latter proceeds
directly, for instance: C_{6}H_{6} + SO_{3} =
C_{6}H_{5}(SO_{2}OH).
As accordingding to the determinations of Thomsen, the heat of
combustion of the _vapours_ of acids RCO_{2} is known where R is a
hydrocarbon, and the heat of combustion of the hydrocarbons R
themselves, it may be seen that the formation of acids, RCO_{2},
from R + CO_{2}, is always accompanied by a _small_ absorption or
development of heat. We give the heats of combustion in thousands
of calories, referred to the molecular weights of the
substances:--
R = H_{2} CH_{4} C_{2}H_{6} C_{6}H_{6}
68·4 212 370 777
RCO_{2} = 69·4 225 387 766
Thus H_{2}, corresponds with formic acid, CH_{2}O_{2}; benzene,
C_{6}H_{6}, with benzoic acid, C_{7}H_{6}O_{2}. The data for the
latter are taken from Stohmann, and refer to the solid condition.
For formic acid Stohmann gives the heat of combustion as 59,000
calories in a liquid state, but in a state of vapour, 64·6
thousand units, which is much less than according to Thomsen.
_Carbonic Oxide._--This gas is formed whenever the combustion of
organic substances takes place in the presence of a large excess of
incandescent charcoal; the air first burns the carbon into carbonic
anhydride, but this in penetrating through the red-hot charcoal is
transformed into carbonic oxide, CO_{2} + C = 2CO. By this reaction
carbonic oxide is prepared by passing carbonic anhydride through
charcoal at a red heat. It may be separated from the excess of carbonic
anhydride by passing it through a solution of alkali, which does not
absorb carbonic oxide. This reduction of carbonic anhydride explains why
carbonic oxide is formed in ordinary clear fires, where the incoming
air passes over a large surface of heated coal. A blue flame is then
observed burning above the coal; this is the burning carbonic oxide.
When charcoal is burnt in stacks, or when a thick layer of coal is
burning in a brazier, and under many similar circumstances, carbonic
oxide is also formed. In metallurgical processes, for instance when
iron is smelted from the ore, very often the same process of conversion
of carbonic anhydride into carbonic oxide occurs, especially if the
combustion of the coal be effected in high, so-called blast, furnaces
and ovens, where the air enters at the lower part and is compelled
to pass through a thick layer of incandescent coal. In this way,
also, combustion with flame may be obtained from those kinds of fuel
which under ordinary conditions burn without flame: for instance,
anthracite, coke, charcoal. Heating by means of a gas-producer--that
is, an apparatus producing combustible carbonic oxide from fuel--is
carried on in the same manner.[19] In transforming one part of charcoal
into carbonic oxide 2,420 heat units are given out, and on burning to
carbonic anhydride 8,080 heat units. It is evident that on transforming
the charcoal first into carbonic oxide we obtain a gas which in burning
is capable of giving out 5,660 heat units for one part of charcoal. This
preparatory transformation of fuel into carbonic oxide, or producer
gas containing a mixture of carbonic oxide (about 1/3 by volume) and
nitrogen (2/3 volume), in many cases presents most important advantages,
as it is easy to completely burn gaseous fuel without an excess of
air, which would lower the temperature.[20] In stoves where solid fuel
is burnt it is impossible to effect the complete combustion of the
various kinds of fuel without admitting an excess of air. Gaseous fuel,
such as carbonic oxide, is easily completely mixed with air and burnt
without excess of it. If, in addition to this, the air and gas required
for the combustion be previously heated by means of the heat which
would otherwise be uselessly carried off in the products of combustion
(smoke)[21] it is easy to reach a high temperature, so high (about
1,800°) that platinum may be melted. Such an arrangement is known as a
_regenerative furnace_.[22] By means of this process not only may the
high temperatures indispensable in many industries be obtained (for
instance, glass-working, steel-melting, &c.), but great advantage
also[23] is gained as regards the quantity of fuel, because the
transmission of heat to the object to be heated, other conditions being
equal, is determined by the difference of temperatures.
[19] [Illustration: FIG. 63.--Gas-producer for the formation of carbon
monoxide for heating purposes.]
In gas-producers all carbonaceous fuels are transformed into
inflammable gas. In those which (on account of their slight
density and large amount of water, or incombustible admixtures
which absorb heat) are not as capable of giving a high temperature
in ordinary furnaces--for instance, fir cones, peat, the lower
kinds of coal, &c.--the same gas is obtained as with the best
kinds of coal, because the water condenses on cooling, and the
ashes and earthy matter remain in the gas-producer. The
construction of a gas-producer is seen from the accompanying
drawing. The fuel lies on the fire-bars O, the air enters through
them and the ash-hole (drawn by the draught of the chimney of the
stove where the gas burns, or else forced by a blowing apparatus),
the quantity of air being exactly regulated by means of valves.
The gases formed are then led by the tube V, provided with a
valve, into the gas main U. The addition of fuel ought to proceed
in such a way as to prevent the generated gas escaping; hence the
space A is kept filled with the combustible material and covered
with a lid.
[20] An excess of air lowers the temperature of combustion, because it
becomes heated itself, as explained in Chapter III. In ordinary
furnaces the excess of air is three or four times greater than the
quantity required for perfect combustion. In the best furnaces
(with fire-bars, regulated air supply, and corresponding chimney
draught) it is necessary to introduce twice as much air as is
necessary, otherwise the smoke contains much carbonic oxide.
[21] If in manufactories it is necessary, for instance, to maintain the
temperature in a furnace at 1,000°, the flame passes out at this
or a higher temperature, and therefore much fuel is lost in the
smoke. For the draught of the chimney a temperature of 100° to
150° is sufficient, and therefore the remaining heat ought to be
utilised. For this purpose the flues are carried under boilers or
other heating apparatus. The preparatory heating of the air is the
best means of utilisation when a high temperature is desired
(_see_ Note 22).
[22] Regenerative furnaces were introduced by the Brothers Siemens
about the year 1860 in many industries, and mark a most important
progress in the use of fuel, especially in obtaining high
temperatures. The principle is as follows: The products of
combustion from the furnace are led into a chamber, I, and heat up
the bricks in it, and then pass into the outlet flue; when the
bricks are at a red heat the products of combustion are passed (by
altering the valves) into another adjoining chamber, II, and air
requisite for the combustion of the generator gases is passed
through I. In passing round about the incandescent bricks the air
is heated, and the bricks are cooled--that is, the heat of the
smoke is returned into the furnace. The air is then passed through
II, and the smoke through I. The regenerative burners for
illuminating gas are founded on this same principle, the products
of combustion heat the incoming air and gas, the temperature is
higher, the light brighter, and an economy of gas is effected.
Absolute perfection in these appliances has, of course, not yet
been attained; further improvement is still possible, but
dissociation imposes a limit because at a certain high temperature
combinations do not ensue, possible temperatures being limited by
reverse reactions. Here, as in a number of other cases, the
further investigation of the matter must prove of direct value
from a practical point of view.
[23] At first sight it appears absurd, useless, and paradoxical to lose
nearly one-third of the heat which fuel can develop, by turning it
into gas. Actually the advantage is enormous, especially for
producing high temperatures, as is already seen from the fact that
fuels rich in oxygen (for instance, wood) when damp are unable,
with any kind of hearth whatever, to give the temperature required
for glass-melting or steel-casting, whilst in the gas-producer
they furnish exactly the same gas as the driest and most
carbonaceous fuel. In order to understand the principle which is
here involved, it is sufficient to remember that a large amount of
heat, but having a low temperature, is in many cases of no use
whatever. We are unable here to enter into all the details of the
complicated matter of the application of fuel, and further
particulars must be sought for in special technical treatises. The
following footnotes, however, contain certain fundamental figures
for calculations concerning combustion.
The transformation of carbonic anhydride, by means of charcoal, into
carbonic oxide (C + CO_{2} = CO + CO) is considered a reversible
reaction, because at a high temperature the carbonic oxide splits up into
carbon and carbonic anhydride, as Sainte-Claire Deville showed by using
the method of the 'cold and hot tube.' Inside a tube heated in a furnace
another thin metallic (silvered copper) tube is fitted, through which a
constant stream of cold water flows. The carbonic oxide coming into
contact with the heated walls of the exterior tube forms charcoal, and
its minute particles settle in the form of lampblack on the lower side of
the cold tube, and, since they are cooled, do not act further on the
oxygen or carbonic anhydride formed.[24] A series of electric sparks
also decomposes carbonic oxide into carbonic anhydride and carbon, and if
the carbonic anhydride be removed by alkali complete decomposition may be
obtained (Deville).[24 bis] Aqueous vapour, which is so similar to
carbonic anhydride in many respects, acts, at a high temperature, on
charcoal in an exactly similar way, C + H_{2}O = H_{2} + CO. From 2
volumes of carbonic anhydride with charcoal 4 volumes of carbonic oxide
(2 molecules) are obtained, and precisely the same from 2 volumes of
water vapour with charcoal 4 volumes of a gas consisting of hydrogen and
carbonic oxide (H_{2} + CO) are formed. This mixture of combustible gases
is called _water gas_.[25] But aqueous vapour (and only when strongly
superheated, otherwise it cools the charcoal) only acts on charcoal to
form a large amount of carbonic oxide at a very high temperature (at
which carbonic anhydride dissociates); it begins to react at about 500°,
forming carbonic anhydride according to the equation C + 2H_{2}O = CO_{2}
+ 2H_{2}. Besides this, carbonic oxide on splitting up forms carbonic
anhydride, and therefore water gas always contains a mixture[26] in which
hydrogen predominates, the volume of carbonic oxide being comparatively
less, whilst the amount of carbonic anhydride increases as the
temperature of the reaction decreases (generally it is more than 3 per
cent.)
[24] The first product of combustion of charcoal is always carbonic
anhydride, and not carbonic oxide. This is seen from the fact that
with a shallow layer of charcoal (less than a decimetre if the
charcoal be closely packed) carbonic oxide is not formed at all.
It is not even produced with a deep layer of charcoal if the
temperature is not above 500°, and the current of air or oxygen is
very slow. With a rapid current of air the charcoal becomes
red-hot, and the temperature rises, and then carbonic oxide
appears (Lang 1888). Ernst (1891) found that below 995° carbonic
oxide is always accompanied by CO_{2}, and that the formation of
CO_{2} begins about 400°. Naumann and Pistor determined that the
reaction of carbonic anhydride with carbon commences at about
550°, and that between water and carbon at about 500°. At the
latter temperature carbonic anhydride is formed, and only with a
rise of temperature is carbonic oxide formed (Lang) from the
action of the carbonic anhydride on the carbon, and from the
reaction CO_{2} + H_{2} = CO + H_{2}O. Rathke (1881) showed that
at no temperature whatever is the reaction as expressed by the
equation CO_{2} + C = 2CO_{2}, complete; a part of the carbonic
anhydride remains, and Lang determined that at about 1,000° not
less than 3 p.c. of the carbonic anhydride remains untransformed
into carbonic oxide, even after the action has been continued for
several hours. The endothermal reactions, C + 2H_{2}O = CO_{2}
+2H_{2}, and CO + H_{2}O = CO_{2} + H_{2}, are just as incomplete.
This is made clear if we note that on the one hand the
above-mentioned reactions are all reversible, and therefore
bounded by a limit; and, on the other hand, that at about 500°
oxygen begins to combine with hydrogen and carbon, and also that
the lower limits of dissociation of water, carbonic anhydride, and
carbonic oxide lie near one another between 500° and 1,200°. For
water and carbonic oxide the lower limit of the commencement of
dissociation is unknown, but judging from the published data
(according to Le Chatelier, 1888) that of carbonic anhydride may
be taken as about 1,050°. Even at about 200° half the carbonic
anhydride dissociates if the pressure be small, about 0·001
atmosphere. At the atmospheric pressure, not more than 0·05 p.c.
of the carbonic anhydride decomposes. The reason of the influence
of pressure is here evidently that the splitting up of carbonic
anhydride into carbonic oxide and oxygen is accompanied by an
increase in volume (as in the case of the dissociation of nitric
peroxide. _See_ Chapter VI., Note 46). As in stoves and lamps, and
also with explosive substances, the temperature is not higher than
2,000° to 2,500°, it is evident that although the partial pressure
of carbonic anhydride is small, still its dissociation cannot here
be considerable, and probably does not exceed 5 p.c.
[24 bis] Besides which L. Mond (1890) showed that the powder of freshly
reduced metallic nickel (obtained by heating the oxide to redness
in a stream of hydrogen) is able, when heated even to 350°, to
completely decompose carbonic oxide into CO_{2} and carbon, which
remains with the nickel and is easily removed from it by heating
in a stream of air. Here 2CO = CO_{2} + C. It should be remarked
that heat is evolved in this reaction (Note 25), and therefore
that the influence of 'contact' may here play a part. Indeed, this
reaction must be classed among the most remarkable instances of
the influence of contact, especially as metals analogous to Ni (Fe
and Co) do not effect this reaction (_see_ Chapter II., Note 17).
[25] A molecular weight of this gas, or 2 volumes CO (28 grams), on
combustion (forming CO_{2}) gives out 68,000 heat units (Thomsen
67,960 calories). A molecular weight of hydrogen, H_{2} (or 2
volumes), develops on burning into _liquid_ water 69,000 heat
units (according to Thomsen 68,300), but if it forms aqueous
vapour 58,000 heat units. Charcoal, resolving itself by combustion
into the molecular quantity of CO_{2} (2 volumes), develops 97,000
heat units. From the data furnished by these exothermal reactions
it follows: (1) that the oxidation of charcoal into carbonic oxide
develops 29,000 heat units; (2) that the reaction C + CO_{2} = 2CO
_absorbs_ 39,000 heat units; (3) C + H_{2}O = H_{2} + CO _absorbs_
(if the water be in a state of vapour) 29,000 calories, but if the
water be liquid 40,000 calories (almost as much as C + CO_{2});
(4) C + H_{2}O = CO_{2} + 2H_{2} _absorbs_ (if the water be in a
state of vapour) 19,000 heat units; (5) the reaction CO + H_{2}O =
CO_{2} + H_{2} _develops_ 10,000 heat units if the water be in the
state of vapour; and (6) the decomposition expressed by the
equation 2CO = C + CO_{2} (Note 24 bis) is accompanied by the
_evolution_ of 39,000 units of heat.
Hence it follows that 2 volumes of CO or H_{2} burning into CO_{2}
or H_{2}O develop almost the same amount of heat, just as also the
heat effects corresponding with the equations
C + H_{2}O = CO + H_{2}
C + CO_{2} = CO + CO
are nearly equal.
[26] _Water gas_, obtained from steam and charcoal at a white heat,
contains about 50 p.c. of hydrogen, about 40 p.c. of carbonic
oxide, about 5 p.c. of carbonic anhydride, the remainder being
nitrogen from the charcoal and air. Compared with producer gas,
which contains much nitrogen, this is a gas much richer in
combustible matter, and therefore capable of giving high
temperatures, and is for this reason of the greatest utility. If
carbonic anhydride could be as readily obtained in as pure a state
as water, then CO might be prepared directly from CO_{2} + C, and
in that case the utilisation of the heat of the carbon would be
the same as in water gas, because CO evolves as much heat as
H_{2}, and even more if the temperature of the smoke be over 100°,
and the water remains in the form of vapour (Note 25). But
producer gas contains a large proportion of nitrogen, so that its
effective temperature is below that given by water gas; therefore
in places where a particularly high temperature is required (for
instance, for lighting by means of incandescent lime or magnesia,
or for steel melting, &c.), and where the gas can be easily
distributed through pipes, water gas is at present held in high
estimation, but when (in ordinary furnaces, re-heating,
glass-melting, and other furnaces) a very high temperature is not
required, and there is no need to convey the gas in pipes,
producer gas is generally preferred on account of the simplicity
of its preparation, especially as for water gas such a high
temperature is required that the plant soon becomes damaged.
There are numerous systems for making water gas, but the American
patent of T. Lowe is generally used. The gas is prepared in a
cylindrical generator, into which hot air is introduced, in order
to raise the coke in it to a white heat. The products of
combustion containing carbonic oxide are utilised for superheating
steam, which is then passed over the white hot coke. Water gas, or
a mixture of hydrogen and carbonic oxide, is thus obtained.
Water gas is sometimes called '_the fuel of the future_,' because
it is applicable to all purposes, develops a high temperature, and
is therefore available, not only for domestic and industrial uses,
but also for gas-motors and for lighting. For the latter purpose
platinum, lime, magnesia, zirconia, and similar substances (as in
the Drummond light, Chapter III.), are rendered incandescent in
the flame, or else the gas is _carburetted_--that is, mixed with
the vapours of volatile hydrocarbons (generally benzene or
naphtha, naphthalene, or simply naphtha gas), which communicate to
the pale flame of carbonic oxide and hydrogen a great brilliancy,
owing to the high temperature developed by the combustion of the
non-luminous gases. As water gas, possessing these properties, may
be prepared at central works and conveyed in pipes to the
consumers, and as it may be produced from any kind of fuel, and
ought to be much cheaper than ordinary gas, it may as a matter of
fact be expected that in course of time (when experience shall
have determined the cheapest and best way to prepare it) it will
not only supplant ordinary gas, but will with advantage everywhere
replace the ordinary forms of fuel, which in many respects are
inconvenient. At present its consumption spreads principally for
lighting purposes, and for use in gas-engines instead of ordinary
illuminating gas. In some cases Dowson gas is prepared in
producers. This is a mixture of water and producer gases obtained
by passing steam into an ordinary producer (Note 19), when the
temperature of the carbon has become sufficiently high for the
reaction C + H_{2}O = CO + H_{2}.
Metals like iron and zinc which at a red heat are capable of decomposing
water with the formation of hydrogen, also decompose carbonic anhydride
with the formation of carbonic oxide; so both the ordinary products of
complete combustion, water and carbonic anhydride, are very similar in
their reactions, and we shall therefore presently compare hydrogen and
carbonic oxide. The metallic oxides of the above-mentioned metals, when
reduced by charcoal, also give carbonic oxide. Priestley obtained it by
heating charcoal with zinc oxide. As free carbonic anhydride may be
transformed into carbonic oxide, so, in precisely the same way, may that
carbonic acid which is in a state of combination; hence, if magnesium or
barium carbonates (MgCO_{3} or BaCO_{3}) be heated to redness with
charcoal, or iron or zinc, carbonic oxide will be produced--for instance,
it is obtained by heating an intimate mixture of 9 parts of chalk and 1
part of charcoal in a clay retort.