the substance dissolved) at small but increasing concentrations
(per 100 grms. of water), the results obtained by Julio Baroni
(1893) show that the value of M found by the formula may either
increase or decrease. An increase, for instance, takes place in
aqueous solutions of HgCl_{2} (from 255 to 334 instead of 271),
KNO_{3} (57-66 instead of 101), AgNO_{3} (104-107 instead of 170),
K_{2}SO_{4} (55-89 instead of 174), sugar (328-348 instead of
342), &c. On the contrary the calculated value of M decreases as
the concentration increases, for solutions of KCl (40-39 instead
of 74·5), NaCl (33-28 instead of 58·5), NaBr (60-49 instead of
103), &c. In this case (as also for LiCl, NaI, C_{2}H_{3}NaO_{2},
&c.) the value of _i_ (Chapter I., Note 49), or the ratio between
the actual molecular weight and that found by the rise of the
boiling point, was found to increase with the concentration,
_i.e._ to be greater than 1, and to differ more and more from
unity as the strength of the solution becomes greater. For
example, according to Schlamp (1894), for LiCl, with a variation
of from 1·1 to 6·7 grm. LiCl per 100 of water, _i_ varies from
1·63 to 1·89. But for substances of the first series (HgCl_{2},
&c.), although in very dilute solutions _i_ is greater than 1, it
approximates to 1 as the concentration increases, and this is the
normal phenomenon for solutions which do not conduct an electric
current, as, for instance, of sugar. And with certain
electrolytes, such as HgCl_{2}, MgSO_{4}, &c., _i_ exhibits a
similar variation; thus, for HgCl_{2} the value of M is found to
vary between 255 and 334; that is, _i_ (as the molecular weight =
271) varies between 1·06 and 0·81. Hence I do not believe that the
difference between _i_ and unity (for instance, for CaCl_{2}, _i_
is about 3, for KI about 2, and decreases with the concentration)
can at present be placed at the basis of any general chemical
conclusions, and it requires further experimental research. Among
other methods by which the value of _i_ is now determined for
dilute solutions is the study of their electroconductivity,
admitting that _i_ = 1 + _a_(_k_-1), where _a_ = the ratio of the
molecular conductivity to the limiting conductivity corresponding
to an infinitely large dilution (_see_ Physical Chemistry), and
_k_ is the number of ions into which the substance dissolved can
split up. Without entering upon a criticism of this method of
determining _i_, I will only remark that it frequently gives
values of _i_ very close to those found by the depression of the
freezing point and rise of the boiling point; but that this
accordance of results is sometimes very doubtful. Thus for a
solution containing 5·67 grms. CaCl_{2} per 100 grms. of water,
_i_, according to the vapour tension = 2·52, according to the
boiling point = 2·71, according to the electroconductivity = 2·28,
while for solutions in propyl alcohol (Schlamp 1894) _i_ is near
to 1·33. In a word, although these methods of determining the
molecular weight of substances in solution show an undoubted
progress in the general chemical principles of the molecular
theory, there are still many points which require explanation.
We will add certain general relations which apply to these
problems. Isotonic (Chapter I., Note 19) solutions exhibit not
only similar osmotic pressures, but also the same vapour tension,
boiling point and freezing temperature. The osmotic pressure bears
the same relation to the fall of the vapour tension as the
specific gravity of a solution does to the specific gravity of the
vapour of the solvent. The general formulæ underlying the whole
doctrine of the influence of the molecular weight upon the
properties of solutions considered above, are: 1. Raoult in
1886-1890 showed that
((_p_-_p_´)/_p_) × (100/_a_) × (M/_m_) = a constant C
where _p_ and _p_´ are the vapour tensions of the solvent and
substance dissolved, _a_ the amount in grms. of the substance
dissolved per 100 grms. of solvent, M and _m_ the molecular
weights of the substance dissolved and solvent. 2. Raoult and
Recoura in 1890 showed that the constant above C = the ratio of
the actual vapour density _d_´ of the solvent to the theoretical
density _d_ calculated according to the molecular weight. This
deduction may now be considered proved, because both the fall of
tension and the ratio of the vapour densities _d_´/_d_ give, for
water 1·03, for alcohol 1·02, for ether 1·04, for bisulphide of
carbon 1·00, for benzene 1·02, for acetic acid 1·63. 3. By
applying the principles of thermodynamics and calling L_{1} the
latent heat of fusion and T_{1} the absolute (= _t_ + 273)
temperature of fusion of the solvent, and L_{2} and T_{2} the
corresponding values for the boiling point, Van't Hoff in
1886-1890 deduced:--
(Depression of freezing point)/(Rise of boiling point)
= (L_{2}/L_{1}) × (T_{1}^2/T_{2}^2)
Depression of freezing point = (AT_{1}^{2}_a_)/(L_{1}M_{1})
Rise of boiling point = (AT_{2}^{2}_a_)/(L_{2}M_{1})
where A = 0·01988 (or nearly 0·02 as we took it above), _a_ is
the weight in grms. of the substance dissolved per 100 grms. of
the solvent, M_{1} the molecular weight of the dissolved substance
(in the solution), and M the molecular weight of this substance
according to its composition and vapour density, then _i_ =
M/M_{1}. The experimental data and theoretical considerations upon
which these formulæ are based will be found in text-books of
physical and theoretical chemistry.
If 100 gram-molecules of water, _i.e._ 1,800 grms, be taken and _n_
gram-molecules of sugar, C_{12}H_{22}O_{11}, _i.e._ _n_ 342 grms., be
dissolved in them, then the depression _d_, or fall (counting from
0°) of the temperature of the formation of ice will be (according to
Pickering)
_n_ = 0 0·010 0·025 0·100 0·250 1·000
_d_ = 0° 0°·0103 0°·0280 0°·1115 0°·2758 1°·1412
which shows that for high degrees of dilution (up to 0·25_n_) _d_
approximately (estimating the possible errors of experiment at ±0°·005)
= _n_1·10, because then _d_ = 0°, 0°·0110, 0°·0275, 0°·1100, 0°·2750,
1°·1000, and the difference between these figures and the results
of experiment for very dilute solutions is less than the possible
errors of experiment (for _n_ = 1 the difference is already greater)
and therefore for dilute solutions of sugar it may be said that _n_
molecules of sugar in dissolving in 100 molecules of water give a
depression of about 1°·1_n_. Similar data for acetone (Chapter I., Note
49) give a depression of 1°·006_n_ for _n_ molecules of acetone per 100
molecules of water. And in general, for indifferent substances (the
majority of organic bodies) the depression per 100H_{2}O is _nearly
n_1°·1 to _n_1°·0 (ether, for instance, gives the last number), and
consequently in dissolving in 100 grms. of water it is about 18°·0_n_
to 19°·0_n_, taking this rule to apply to the case of a small number
of _n_ (not over 0·2_n_). If instead of water, other liquid or fused
solvents (for example, benzene, acetic acid, acetone, nitrobenzene or
molten naphthaline, metals, &c.) be taken and in the proportion of 100
molecules of the solvent to _n_ molecules of a dissolved indifferent
(neither acid nor saline) substance, then the depression is found to be
equal to from 0°·62_n_ to 0°·65_n_ and in general K_n_. If the molecular
weight of the solvent = _m_, then 100 gram-molecules will weigh 100_m_
grms., and the depression will be approximately (taking 0·63_n_) equal
to _m_0·63_n_ degrees for _n_ molecules of the substance dissolved in
100 grms. of the solvent, or in general the depression for 100 grms.
of a given solvent = _kn_ where _k_ is almost a constant quantity (for
water nearly 18, for acetone nearly 37, &c.) for all dilute solutions.
Thus, having found a convenient solvent for a given substance and
prepared a definite (by weight) solution (_i.e._ knowing how many grms.
_r_ of the solvent there are to _q_ grms. of the substance dissolved)
and having determined the depression _d_--_i.e._ the fall in temperature
of freezing for the solvent--it is possible to determine the molecular
weight of the substance dissolved, because _d_ = _kn_ where _d_ is found
by experiment and _k_ is determined by the nature of the solvent, and
therefore _n_ or the number of molecules of the substance dissolved can
be found. But if _r_ grms. of the solvent and _q_ grms. of the substance
dissolved are taken, then there are 100_q_/_r_ of the latter per 100
grms. of the former, and this quantity = _n_X, where _n_ is found from
the depression and = _d_/_k_ and X is the molecular weight of the
substance dissolved. Hence X = 100_qk_/_rd_, which gives the molecular
weight, naturally only approximately, but still with sufficient accuracy
to easily indicate, for instance, whether in peroxide of hydrogen the
molecule contains HO or H_{2}O_{2} or H_{3}O_{3}, &c. (H_{2}O_{2} is
obtained). Moreover, attention should be drawn to the fact that a great
many substances taken as solvents give per 100 molecules a depression of
about 0·63_n_, whilst water gives about 1·05_n_, _i.e._ a larger
quantity, as though the molecules of liquid water were more complex than
is expressed by the formula H_{2}O.[28] A similar phenomenon which
repeats itself in the osmotic pressure, vapour tension of the solvent,
&c. (_see_ Chapter I., Notes 19 and 49), _i.e._ a variation of the
constant (_k_ for 100 grms. of the solvent or K for 100 molecules of it),
is also observed in passing from indifferent substances to saline (to
acids, alkalis and salts) both in aqueous and other solutions as we will
show (according to Pickering's data 1892) for solutions of NaCl and
CuSO_{4} in water. For
_n_ = 0·01 0·03 0·05 0·1 0·5
molecules of NaCl the depression is
_d_ = 0°·0177 0°·0598 0°·0992 0°·1958 0°·9544
which corresponds to a depression per molecule
K = 1·77 1·96 1·98 1·96 1·91
_i.e._ here in the most dilute solutions (when _n_ is nearly 0) _d_ is
obtained about 1·7_n_, while in the case of sugar it was about 1·1_n_.
For CuSO_{4} for the same values of _n_, experiment gave:
_d_ = 0°·0164 0°·0451 0°·0621 0°·1321 0°·5245
K = 1·64 1·50 1·44 1·32 1·05
_i.e._ here again _d_ for very dilute solutions is nearly 1·7_n_, but
the value of K falls as the solution becomes more concentrated, while
for NaCl it at first increased and only fell for the more concentrated
solutions. The value of K in the solution of _n_ molecules of a body
in 100H_{2}O, when _d_ = K_n_, for very dilute solutions of CaCl_{2}
is nearly 2·6, for Ca(NO_{3})_{2} nearly 2·5, for HNO_{3}, KI and KHO
nearly 1·9-2·O, for borax Na_{2}B_{4}O_{7} nearly 3·7, &c., while for
sugar and similar substances it is, as has been already mentioned,
nearly 1·0-1·1. Although these figures are very different[28 bis] still
_k_ and K may be considered constant for analogous substances, and
therefore the weight of the molecule of the body in solution can be
found from _d_. And as the vapour tension of solutions and their boiling
points (_see_ Note 27 bis and Chapter I., Note 51) vary in the same
manner as the freezing point depression, so they also may serve as means
for determining the molecular weight of a substance in solution.[29]
[28] A similar conclusion respecting the molecular weight of liquid
water (_i.e._ that its molecule in a liquid state is more
complex than in a gaseous state, or polymerized into H_{8}O_{4},
H_{6}O_{3} or in general into _n_H_{2}O) is frequently met
in chemico-physical literature, but as yet there is no basis
for its being fully admitted, although it is possible that a
polymerization or aggregation of several molecules into one
takes place in the passage of water into a liquid or solid
state, and that there is a converse depolymerization in the
act of evaporation. Recently, particular attention has been
drawn to this subject owing to the researches of Eötvös (1886)
and Ramsay and Shields (1893) on the variation of the surface
tension N with the temperature (N = the capillary constant _a_^2
multiplied by the specific gravity and divided by 2, for example,
for water at 0° and 100° the value of _a_^2 = 15·41 and 12·58
sq. mm., and the surface tension 7·92 and 6·04). Starting from
the absolute boiling point (Chapter II., Note 29) and adding 6°,
as was necessary from all the data obtained, and calling this
temperature T, it is found that AS = _k_T, where S is the surface
of a gram-molecule of the liquid (if M is its weight in grams,
_s_ its sp. gr., then its sp. volume = M/_s_, and the surface
S = [3root](M/_s_)^2), A the surface tension (determined by
experiment at T), and _k_ a constant which is independent of
the composition of the molecule. The equation AS = _k_T is in
complete agreement with the well-known equation for gases _vp_
= RT (p. 140) which serves for deducing the molecular weight
from the vapour density. Ramsay's researches led him to the
conclusion that the liquid molecules of CS_{2}, ether, benzene,
and of many other substances, have the same value as in a state
of vapour, whilst with other liquids this is not the case, and
that to obtain an accordance, that is, that _k_ shall be a
constant, it is necessary to assume the molecular weight in the
liquid state to be _n_ times as great. For the fatty alcohols and
acids _n_ varies from 1-1/2 to 3-1/2, for water from 2-1/4 to 4,
according to the temperature (at which the depolymerization takes
place). Hence, although this subject offers a great theoretical
interest, it cannot be regarded as firmly established, the more
so since the fundamental observations are difficult to make and
not sufficiently numerous; should, however, further experiments
confirm the conclusions arrived at by Professor Ramsay, this will
give another method of determining molecular weights.
[28 bis] Their variance is expressed in the same manner as was done by
Van't Hoff (Chapter I., Notes 19 and 49) by the quantity _i_,
taking it as = 1 when _k_ = 1·05, in that case for KI, _i_ is
nearly 2, for borax about 4, &c.
[29] We will cite one more example, showing the direct dependence of
the properties of a substance on the molecular weight. If one
molecular part by weight of the various chlorides--for instance,
of sodium, calcium, barium, &c.--be dissolved in 200 molecular
parts by weight of water (for instance, in 3,600 grams) then
it is found that the greater the molecular weight of the salt
dissolved, the greater is the specific gravity of the resultant
solution.
Molecular Sp. gr. Molecular Sp. gr.
weight at 15° weight at 15°
HCl 36·5 1·0041 CaCl_{2} 111 1·0236
NaCl 58·5 1·0106 NiCl_{2} 130 1·0328
KCl 74·5 1·0121 ZnCl_{2} 136 1·0331
BeCl_{2} 80 1·0138 BaCl_{2} 208 1·0489
MgCl_{2} 95 1·0203
Thus not only in vapours and gases, but also in dilute solutions
of solid and liquid substances, we see that if not all, still many
properties are wholly dependent upon the molecular weight and not upon
the quality of a substance, and that this gives the possibility of
determining the weight of molecules by studying these properties (for
instance, the vapour density, depression of the freezing point, &c.) It
is apparent from the foregoing that the physical and even more so the
chemical properties of homogeneous substances, more especially solid and
liquid, do not depend exclusively upon the weights of their molecules,
but that many are in definite (_see_ Chapter XV.) dependence upon the
weights of the atoms of the elements entering into their composition,
and are determined by their quantitative and individual peculiarities.
Thus the density of solids and liquids (as will afterwards be shown) is
chiefly determined by the weights of the atoms of the elements entering
into their composition, inasmuch as dense elements (in a free state) and
compounds are only met with among substances containing elements with
large atomic weights, such as gold, platinum, and uranium. And these
elements themselves, in a free state, are the heaviest of all elements.
Substances containing such light elements as hydrogen, carbon, oxygen
and nitrogen (like many organic substances) never have a high specific
gravity; in the majority of cases it scarcely exceeds that of water. The
density generally decreases with the increase of the amount of hydrogen,
as the lightest element, and a substance is often obtained lighter than
water. The refractive power of substances also entirely depends on the
composition and the properties of the component elements.[29 bis] The
history of chemistry presents a striking example in point--Newton
foresaw from the high refractive index of the diamond that it would
contain a combustible substance since so many combustible oils have a
high refractive power. We shall afterwards see (Chapter XV.) that many
of those properties of substances which are in direct dependence not
upon the weight of the molecules but upon their composition, or, in
other words, upon the properties and quantities of the elements entering
into them, stand in a peculiar (periodic) dependence upon the atomic
weight of the elements; that is, the mass (of molecules and atoms),
proportional to the weight, determines the properties of substances
as it also determines (with the distance) the motions of the heavenly
bodies.
[29 bis] With respect to the optical refractive power of substances, it
must first be observed that the coefficient of refraction is
determined by two methods: (_a_) either all the data are referred
to one definite ray--for instance, to the Fraunhofer (sodium) line
D of the solar spectrum--that is, to a ray of definite wave
length, and often to that red ray (of the hydrogen spectrum) whose
wave length is 656 millionths of a millimetre; (_b_) or Cauchy's
formula is used, showing the relation between the coefficient of
refraction and dispersion to the wave length _n_ = A +
(B/([Greek: l]^2)), where A and B are two constants varying for
every substance but constant for all rays of the spectrum, and
[Greek: l] is the wave length of that ray whose coefficient of
refraction is _n_. In the latter method the investigation usually
concerns the magnitudes of A, which are independent of dispersion.
We shall afterwards cite the data, investigated by the first
method, by which Gladstone, Landolt, and others established the
conception of the refraction equivalent.
It has long been known that the _coefficient of refraction n_
for a given substance decreases with the density of a substance D,
so that the magnitude (_n_-1) ÷ D = C is almost constant for a
given ray (having a definite wave length) and for a given
substance. This constant is called the _refractive energy_, and
its product with the atomic or molecular weight of a substance the
_refraction equivalent_. The coefficient of refraction of oxygen
is 1·00021, of hydrogen 1·00014, their densities (referred to
water) are 0·00143 and 0·00009, and their atomic weights, O = 16,
H = 1; hence their refraction equivalents are 3 and 1·5. Water
contains H_{2}O, consequently the sum of the equivalents of
refraction is (2 × 1·5) + 3 = 6. But as the coefficient of
refraction of water = 1·331, its refraction equivalent = 5·958, or
nearly 6. Comparison shows that, approximately, the sum of the
refraction equivalents of the atoms forming compounds (or
mixtures) is equal to the refraction equivalent of the compound.
According to the researches of Gladstone, Landolt, Hagen, Brühl
and others, the refraction equivalents of the elements are--H =
1·3, Li = 3·8, B = 4·0, C = 5·0, N = 4·1 (in its highest state of
oxidation, 5·3), O = 2·9, F = 1·4, Na = 4·8, Mg = 7·0, Al = 8·4,
Si = 6·8, P = 18·3, S = 16·0, Cl = 9·9, K = 8·1, Ca = 10·4, Mn =
12·2, Fe = 12·0 (in the salts of its higher oxides, 20·1), Co =
10·8, Cu = 11·6, Zn = 10·2, As = 15·4, Bi = 15·3, Ag = 15·7, Cd =
13·6, I = 24·5, Pt = 26·0, Hg = 20·2, Pb = 24·8, &c. The
refraction equivalents of many elements could only be calculated
from the solutions of their compounds. The composition of a
solution being known it is possible to calculate the refraction
equivalent of one of its component parts, those for all its other
components being known. The results are founded on the acceptance
of a law which cannot be strictly applied. Nevertheless the
representation of the refraction equivalents gives an easy means
for directly, although only approximately, obtaining the
coefficient of refraction from the chemical composition of a
substance. For instance, the composition of carbon bisulphide is
CS_{2} = 76, and from its density, 1·27, we find its coefficient
of refraction to be 1·618 (because the refraction equivalent = 5 +
2 × 16 = 37), which is very near the actual figure. It is evident
that in the above representation compounds are looked on as simple
mixtures of atoms, and the physical properties of a compound as
the sum of the properties present in the elementary atoms forming
it. If this representation of the presence of simple atoms in
compounds had not existed, the idea of combining by a few figures
a whole mass of data relating to the coefficient of refraction of
different substances could hardly have arisen. For further details
on this subject, see works on _Physical Chemistry_.
CHAPTER VIII
CARBON AND THE HYDROCARBONS
It is necessary to clearly distinguish between the two closely-allied
terms, charcoal and carbon. Charcoal is well known to everybody,
although it is no easy matter to obtain it in a chemically pure state.
Pure charcoal is a simple, insoluble, infusible, combustible substance
produced by heating organic matter, and has the familiar aspect of
a black mass, devoid of any crystalline structure, and completely
insoluble. Charcoal is a substance possessing a peculiar combination
of physical and chemical properties. This substance, whilst in a state
of ignition, combines directly with oxygen; in organic substances it
is found in combination with hydrogen, oxygen, nitrogen, and sulphur.
But in all these combinations there is no real charcoal, as in the same
sense there is no ice in steam. What is found in such combinations
is termed 'carbon'--that is, an element common to charcoal, to those
substances which can be formed from it, and also to those substances
from which it can be obtained. Carbon may take the form of charcoal, but
occurs also as diamond and as graphite. Truly no other element has such
a wide terminology. Oxygen is always called 'oxygen,' whether it is in a
free gaseous state, or in the form of ozone, or oxygen in water, or in
nitric acid or in carbonic anhydride. But here there is some confusion.
In water it is evident that there is no oxygen in a gaseous form, such
as can be obtained in a free state, no oxygen in the form of ozone, but
a substance which is capable of producing both oxygen, ozone, and water.
As an element, oxygen possesses a known chemical individuality, and an
influence on the properties of those combinations into which it enters.
Hydrogen gas is a substance which reacts with difficulty, but the
element hydrogen represents in its combinations an easily displaceable
component part. Carbon may be considered as an atom of carbon matter,
and charcoal as a collection of such atoms forming a whole substance,
or mass of molecules of the substance. The accepted atomic weight
of carbon is 12, because that is the least quantity of carbon which
enters into combination in molecules of its compounds; but the weight
of the molecules of charcoal is probably very much greater. This weight
remains unknown because charcoal is capable of but few direct reactions
and those only at a high temperature (when the weight of its molecules
probably changes, as when ozone changes into oxygen), and it does not
turn into vapour. Carbon exists in nature, both in a free and combined
state, in most varied forms and aspects. Carbon in a free state is
found in at least three different forms, as charcoal, graphite, and the
diamond. In a combined state it enters into the composition of what are
called organic substances--a multitude of substances which are found in
all plants and animals. It exists as carbonic anhydride both in air and
in water, and in the soil and crust of the earth as salts of carbonic
acid and as organic remains.
The variety of the substances of which the structure of plants and
animals is built up is familiar to all. Wax, oil, turpentine, and
tar, cotton and albumin, the tissue of plants and the muscular fibre
of animals, vinegar and starch, are all vegetable and animal matters,
and all carbon compounds.[1] The class of carbon compounds is so vast
that it forms a separate branch of chemistry, known under the name of
organic chemistry--that is, the chemistry of carbon compounds, or, more
strictly, of the hydrocarbons and their derivatives.
[1] Wood is the non-vital part of ligneous plants: the vital part of
ordinary trees is situated between the bark and the lignin. Every
year a layer of lignin is deposited on this part by the juices
which are absorbed by the roots and drawn up by the leaves; for
this reason the age of trees may be determined by the number of
lignin layers deposited. The woody matter consists principally
of fibrous tissue on to which the lignin or so-called incrusting
matter has been deposited. The tissue has the composition
C_{6}H_{10}O_{5}, the substance deposited on it contains more
carbon and hydrogen and less oxygen. This matter is saturated
with moisture when the wood is in a fresh state. Fresh birch wood
contains about 31 p.c. of water, lime wood 47 p.c., oak 35 p.c.,
pine and fir about 37 p.c. When dried in the air the wood loses a
considerable quantity of water and not more than 19 p.c. remains.
By artificial means this loss of water may be increased. If water
be driven into the pores of wood the latter becomes heavier than
water, as the lignin of which it is composed has a density of
about 1·6. One cubic centimetre of birch wood does not weigh more
than 0·901 gram, fir 0·894, lime tree 0·817, poplar 0·765 when
in a fresh state; when in a dry state birch weighs 0·622, pine
0·550, fir 0·355, lime 0·430, guaiacum 1·342, ebony 1·226. On one
hectare (2·7 acres) of woodland the yearly growth averages the
amount of 3,000 kilograms (or about 3 tons) of wood, but rarely
reaches as much as 5,000 kilos. The average chemical composition
of wood dried in air may be expressed as follows:--Hygroscopic
water 15 p.c., carbon 42 p.c., hydrogen 5 p.c., oxygen and
nitrogen 37 p.c., ash 1 p.c. Wood parts with its hygroscopic water
at 150°, and decomposes at about 300°, giving a brown, brittle,
so-called red charcoal; above 350° black charcoal is produced.
As the hydrogen contained in wood requires for its combustion
about forty parts by weight of oxygen, which is present to the
amount of about 36 p.c., all that burns of the wood is the carbon
which it contains, 100 parts of wood only giving out as much heat
as forty parts of charcoal, and therefore it would be far more
profitable to use charcoal for heating purposes than wood, if it
were possible to obtain it in such quantities as correspond with
its percentage ratio--that is forty parts per 100 parts of wood.
Generally, however, the quantity produced is far less, not more
than 30 p.c., because part of the carbon is given off as gas, tar,
&c. If wood has to be transported great distances, or if it is
necessary to obtain a very high temperature by burning it, then
even as little as 25 p.c. of charcoal from 100 parts of wood may be
advantageous. Charcoal (from wood) develops on burning 8,000 heat
units, whilst wood dried in air does not develop more than 2,800
units of heat; therefore seven parts of charcoal give as much heat
as twenty parts of wood. As regards the temperature of combustion,
it is far higher with charcoal than with wood, because twenty parts
of burning wood give, besides the carbonic anhydride which is also
formed together with charcoal, eleven parts of water, the
evaporation of which requires a considerable amount of heat.
[Illustration: FIG. 57.--Apparatus for the dry distillation of
wood. The retort _a_ containing the wood is heated by the flues _c
e_. The steam and volatile products of distillation pass along the
tube g through the condenser _m_, where they are condensed. The
form, distribution, and dimensions of the apparatus vary.]
The composition of the growing parts of plants, the leaves, young
branches, shoots, &c., differs from the composition of the wood
in that these vital parts contain a considerable quantity of sap
which contains much nitrogenous matter (in the wood itself there
is very little), mineral salts, and a large amount of water.
Taking, for example, the composition of clover and pasture hay in
the green and dry state; in 100 parts of green clover there is
about 80 p.c. of water and 20 p.c. of dry matter, in which there
are about 3·5 parts of nitrogenous albuminous matter, about 9·5
parts of soluble and about 5 parts of insoluble non-nitrogenous
matter, and about 2 p.c. of ash. In dry clover or clover-hay there
is about 15 p.c. of water, 13 p.c. of nitrogenous matter, and 7
p.c. of ash. This composition of grassy substances shows that they
are capable of forming the same sort of charcoal as wood itself.
It also shows the difference of nutritive properties existing
between wood and the substances mentioned. These latter serve as
food for animals, because they contain those substances which are
capable of being dissolved (entering into the blood) and forming
the body of animals; such substances are proteids, starch, &c. Let
us remark here that with a good harvest an acre of land gives in
the form of grass as much organic substance as it yields in the
form of wood.
One hundred parts of dry wood are capable of giving, on dry
distillation, besides 25 p.c. of charcoal and 10 p.c. or more of
tar, 40 p.c. of watery liquid, containing acetic acid and wood
spirit, and about 25 p.c. of gases, which may be used for heating
or lighting purposes, because they do not differ from ordinary
illuminating gas, which can indeed be obtained from wood. As
wood-charcoal and tar are valuable products, in some cases the
dry distillation of wood is carried on principally for producing
them. For this purpose those kinds of woods are particularly
advantageous which contain resinous substances, especially
coniferous trees, such as fir, pine, &c.; birch, oak, and ash
give much less tar, but on the other hand they yield more aqueous
liquor. The latter is used for the manufacture of wood spirit,
CH_{4}O, and acetic acid, C_{2}H_{4}O_{2}. In such cases, the
dry distillation is carried on in stills. The stills are nothing
more than horizontal or vertical cylindrical retorts, made of
boiler plate, heated with fuel and having apertures at the top
and sometimes also at the bottom for the exit of the light and
heavy products of distillation. The dry distillation of wood in
stoves is carried on in two ways, either by burning a portion of
the wood inside the stove in order to submit the remainder to dry
distillation by means of the heat obtained in this manner, or by
placing the wood in a stove the thin sides of which are surrounded
with a flue leading from the fuel, placed in a space below.
The first method does not give such a large amount of liquid
products of the dry distillation as the latter. In the latter
process there is generally an outlet below for emptying out the
charcoal at the close of the operation. For the dry distillation
of 100 parts of wood from forty to twenty parts of fuel are used.
In the north of Russia wood is so plentiful and cheap that this
locality is admirably fitted to become the centre of a general
trade in the products of its dry distillation. Coal (Note 6),
sea-weed, turf, animal substances (Chapter VI.), &c., are also
submitted to the process of dry distillation.
If any one of these organic compounds be strongly heated without free
access of air--or, better still, in a vacuum--it decomposes with more or
less facility. If the supply of air be insufficient, or the temperature
be too low for combustion (_see_ Chapter III.), and if the first
volatile products of transformation of the organic matter are subjected
to condensation (for example, if the door of a stove be opened), an
imperfect combustion takes place, and smoke, with charcoal or soot,
is formed.[2] The nature of the phenomenon, and the products arising
from it, are the same as those produced by heating alone, since that
part which is in a state of combustion serves to heat the remainder of
the fuel. The decomposition which takes place on heating a compound
composed of carbon, hydrogen, and oxygen is as follows:--A part of the
hydrogen is separated in a gaseous state, another part in combination
with oxygen, and a third part separates in combination with carbon, and
sometimes in combination with carbon and oxygen in the form of gaseous
or volatile products, or, as they are also called, the products of
dry distillation. If the vapours of these products are passed through
a strongly heated tube, they are changed again in a similar manner
and finally resolve themselves into hydrogen and charcoal. Altogether
these various products of decomposition contain a smaller amount of
carbon than the original organic matter; part of the carbon remains
in a free state, forming charcoal.[3] It remains in that space where
the decomposition took place, in the shape of the black, infusible,
non-volatile charcoal familiar to all. The earthy matter and all
non-volatile substances (ash) forming a part of the organic matter,
remain behind with the charcoal. The tar-like substances, which require
a high temperature in order to decompose them, also remain mixed with
charcoal. If a volatile organic substance, such as a gaseous compound
containing oxygen and hydrogen, be taken, the carbon separates on
passing the vapour through a tube heated to a high temperature. Organic
substances when burning with an insufficient supply of air give off
soot--that is, charcoal--proceeding from carbon compounds in a state of
vapour, the hydrogen of which has, by combustion, been converted into
water; so, for instance, turpentine, naphthalene, and other hydrocarbons
which are with difficulty decomposed by heat, easily yield carbon in the
form of soot during combustion. Chlorine and other substances which,
like oxygen, are capable of taking up hydrogen, and also substances
which are capable of taking up water, can also separate carbon from (or
char) most organic substances.
[2] The result of imperfect combustion is not only the loss of a part
of the fuel and the production of smoke, which in some respects
is inconvenient and injurious to health, but also a low flame
temperature, which means that a less amount of heat is transmitted
to the object heated. Imperfect combustion is not only always
accompanied by the formation of soot or unburnt particles of
charcoal, but also by that of carbonic oxide, CO, in the smoke
(Chapter IX.) which burns, emitting much heat. In works and
factories where large quantities of fuel are consumed, many
appliances are adopted to ensure perfect combustion, and to combat
against such a ruinous practice as the imperfect combustion of
fuel. The most effective and radical means consists in employing
combustible gases (producer and water gases), because by their
aid perfect combustion can be easily realised without a loss of
heat-producing power and the highest temperature can be reached.
When solid fuel is used (such as coal, wood, and turf), imperfect
combustion is most liable to occur when the furnace doors are
opened for the introduction of fresh fuel. The step furnace may
often prove a remedy for this defect. In the ordinary furnace
fresh fuel is placed on the burning fuel, and the products of dry
distillation of the fresh fuel have to burn at the expense of
the oxygen remaining uncombined with the burnt fuel. Imperfect
combustion is observed in this case also from the fact that the
dry distillation and evaporation of the water of the fresh fuel
lying on the top of that burnt, lowers the temperature of the
flame, because part of the heat becomes latent. On this account a
large amount of smoke (imperfect combustion) is observed when a
fresh quantity of fuel is introduced into the furnace. This may
be obviated by constructing the furnace (or managing the stoking)
in such a way that the products of distillation pass through
the red-hot charcoal remaining from the burnt fuel. It is only
necessary in order to ensure this to allow a sufficient quantity
of air for perfect combustion. All this may be easily attained
by the use of step fire-bars. The fuel is fed into a hopper and
falls on to the fire-bars, which are arranged in the form of a
staircase. The burning charcoal is below, and hence the flame
formed by the fresh fuel is heated by the contact of the red-hot
burning charcoal. An air supply through the fire grate, an equal
distribution of the fuel on the fire-bars (otherwise the air will
blow through empty spaces and lower the temperature), a proper
proportion between the supply of air and the chimney draught,
and a perfect admixture of air with the flame (without an undue
excess of air), are the means by which we can contend against the
imperfect combustion of such kinds of fuel as wood, peat, and
ordinary (smoky) coal. Coke, charcoal, anthracite, burn without
smoke, because they do not contain hydrogenous substances which
furnish the products of dry distillation, but imperfect combustion
may occur with them also; in that case the smoke contains carbonic
oxide.
[3] Under the action of air, organic substances are capable of
oxidising to such an extent that all the carbon and all the
hydrogen they contain will be transformed into carbonic anhydride
and water. The refuse of plants and that of animals are subjected
to such a change whether they slowly decompose and putrefy, or
rapidly burn with direct access of air. But if the supply of air
be limited, there can be no complete transformation into water and
carbonic anhydride, there will be other volatile matters (rich in
hydrogen), while charcoal must remain as a non-volatile substance.
All organic substances are unstable, they do not resist heat, and
change even at ordinary temperatures, particularly if water be
present. It is therefore easy to understand that charcoal may in
many cases be obtained through the transformation of substances
entering into the composition of organisms, but that it is never
found in a pure state.
However, water and carbonic anhydride are not the only products
separated from organic substances. Carbon, hydrogen, and oxygen
are capable of giving a multitude of compounds; some of these
are volatile compounds, gaseous, soluble in water--they are
carried off from organic matter, undergoing change without access
of air. Others, on the contrary, are non-volatile, rich in
carbon, unaffected by heat and other agents. The latter remain
in admixture with charcoal in the place where the decomposition
takes place; such, for example, are tarry substances. The quantity
of those bodies which are found mixed with the charcoal is very
varied, and depends on the energy and duration of the decomposing
agent. The annexed table shows, according to the data of Violette,
those changes which wood undergoes at various temperatures when
submitted to dry distillation by means of superheated steam:--
+----------------------------+------------------------------+
| Residue | |
|Temperature from 100 parts | In 100 parts of the residual |
| of alder wood | charcoal |
+----------------------------+---------------------+--------+
| | C H O and N | Ash |
| 150° 100·0 | 47·5 6·1 46·3 | 0·1 |
| 350° 29·7 | 76·6 4·1 18·4 | 0·6 |
| 1032° 18·7 | 81·9 2·3 14·1 | 1·6 |
| 1500° 17·3 | 95·0 0·7 3·8 | 1·7 |
+----------------------------+---------------------+--------+
Wood charcoal is prepared in large quantities in a similar manner--that
is, by the partial combustion of wood.[4] In nature a similar process
of carbonisation of vegetable refuse takes place in its transformation
under water, as shown by the marshy vegetation which forms peat.[5] In
this manner doubtless the enormous masses of coal were formed[6] which,
following the example set by England, are now utilised everywhere as
the principal material for heating steam boilers, and in general for
all purposes of heating and burning.[7] Russia possesses many very
rich coalfields, amongst which the Donetz district is most worthy of
remark.[8]
[4] The object of producing charcoal from wood has been explained in
Note 1. _Wood charcoal_ is obtained in so-called stacks by
partially burning the wood, or by means of dry distillation (Note
1) without the access of air. It is principally manufactured for
metallurgical processes, especially for smelting and forging
iron. The preparation of charcoal in stacks has one advantage,
and that is that it may be done on any spot in the forest. But
in this way all the products of dry distillation are lost.
For charcoal burning, a pile or stack is generally built, in
which the logs are placed close together, either horizontally,
vertically, or inclined, forming a stack of from six to fifty
feet in diameter and even larger. Under the stack are several
horizontal air passages, and an opening in the middle to let out
the smoke. The surface of the stack is covered with earth and
sods to a considerable thickness, especially the upper part, in
order to hinder the free passage of air and to concentrate the
heat inside. When the stack is kindled, the pile begins to settle
down by degrees, and it is then necessary to look after the turf
casing and keep it in repair. As the combustion spreads throughout
the whole pile, the temperature rises and real dry distillation
commences. It is then necessary to stop the air holes, in order as
much as possible to prevent unnecessary combustion. The nature of
the process is, that part of the fuel burns and develops the heat
required for subjecting the remainder to dry distillation. The
charring is stopped when the products of dry distillation, which
are emitted, no longer burn with a brilliant flame, but the pale
blue flame of carbonic oxide appears. Dry wood in stacks yields
about one-fourth of its weight of charcoal.
[5] When dead vegetable matter undergoes transformation in air, in the
presence of moisture and lower organisms, there remains a
substance much richer in carbon--namely, humus, black earth or
mould. 100 parts of humus in a dry state contain about 70 p.c.
of carbon. The roots, leaves, and stems of plants which wither
and fall to the ground form a soil rich in humus. The non-vital
vegetable substances (ligneous tissue) first form brown matter
(ulmic compounds), and then black matter (humic substances),
which are both insoluble in water; after this a brown acid is
produced, which is soluble in water (apocrenic acid), and lastly
a colourless acid also soluble in water (crenic acid). Alkali
dissolves a part of the original brown and black substances,
forming solutions of a brown tint (ulmic and humic acids) which
sometimes communicate their colour to springs and rivers. The
proportion of humus in soil generally has a direct influence on
its fertility; firstly, because putrefying plants develop carbonic
anhydride and ammonia, and yield the substances forming the ashes
of plants, which are necessary to vegetation; secondly, because
humus is capable of attracting the moisture of the air and of
absorbing water (twice its weight) and in this way keeps the soil
in a damp condition, which is indispensable for nourishment;
thirdly, humus renders the soil porous, and, fourthly, it renders
it more capable of absorbing the heat of the sun's rays. On this
account black earth is often most remarkable for its fertility.
One object of manuring is to increase the quantity of humus in the
soil, and any easily changeable vegetable or any animal matter
(composts) may be used. The boundless tracts of black earth soil
in Russia are capable of bestowing countless wealth on the country.
The origin and extent of black earth soil are treated in detail in
Professor Dokouchaeff's works.
If those substances which produce humus undergo decomposition
under water, less carbonic anhydride is formed, a quantity of
marsh gas, CH_{4}, is evolved, and the solid residue forms
an acid humus found in great quantities in marshy places and
called _peat_. Peat is especially abundant in the lowlands of
Holland, North Germany, Ireland, and Bavaria. In Russia it is
likewise found in large quantities, especially in the North-West
districts. The old hard forms of peat resemble in composition and
properties brown coal; the newest formations, as yet unhardened
by pressure, form very porous masses which retain traces of the
vegetable matter from which they have been formed. Dried (and
sometimes pressed) peat is used as fuel. The composition of peat
varies considerably with the locality in which it is found. When
dried in air it does not contain less than 15 p.c. of water and
8 p.c. of ash; the remainder consists of 45 p.c. of carbon, 4
p.c. of hydrogen, 1 p.c. of nitrogen, and 28 p.c. of oxygen. Its
heating power is about equivalent to that of wood. The brown
earthy varieties of coal were probably formed from peat. In other
cases they have a marked woody structure, and are then known as
lignites. The composition of the brown sorts of coal resembles in
a marked degree that of peat--namely, in a dried state brown coal
contains on an average 60 p.c. of carbon, 5 p.c. of hydrogen, 26
p.c. of oxygen and nitrogen, and 9 p.c. of ash. In Russia brown
coal is met with in many districts near Moscow, in the Governments
of Toula and Tver and the neighbourhood; it is very usually used
as fuel, particularly when found in thick seams. The brown coals
usually burn with a flame like wood and peat, and are akin to them
in heating power, which is half or a third that of the best coal.
[6] Grass and wood, the vegetation of primæval seas and similar refuse
of all geological periods, must have been in many cases subjected
to the same changes they now undergo--that is, under water they
formed peat and lignites. Such substances, preserved or a long
time underground, subjected to the action of water, compressed by
the new strata formed above them, transformed by the separation
of their more volatile component parts (peat and lignites, even
in their last condition, still continue to evolve nitrogen,
carbonic anhydride, and marsh gases) form _coal_. Coal is a
dense homogeneous mass, black, with an oily or glassy lustre, or
more rarely dull without any evident vegetable structure; this
distinguishes it in appearance from the majority of lignites.
The density of coal (not counting the admixture of pyrites,
&c.) varies from 1·25 (dry bituminous coal) to 1·6 (anthracite,
flameless), and even reaches 1·9 in the very dense variety of
coal found in the Olonetzky government (termed shungite), which
according to the investigations of Professor Inostrantzeff may be
regarded as the extreme member of the various forms of coal.
In order to explain the formation of coal from vegetable matter,
Cagniard de la Tour enclosed pieces of dried wood in a tube and
heated them to the boiling point of mercury, when the wood was
changed into a semi-liquid black mass from which a substance
exceedingly like coal separated. In this manner some kinds of
wood formed coal which on being heated left caking coke, others
non-caking; precisely as we find with the natural varieties of
coal. Violette repeated these experiments with wood dried at
150°, and showed that when wood is decomposed in this way, a
gas, an aqueous liquor, and a residue are formed. The latter
at a temperature of 200° has the properties of wood charcoal
incompletely burnt; at 300° and higher a homogeneous mass like
coal is formed which at 340° is dense and without cavities. At
400° the residue resembles anthracite. In nature probably the
decomposition was in rare cases effected by heat alone; more
generally it was effected by means of water and heat, but in
either case the result ought to be almost the same.
The average composition of coal compiled from many analyses,
disregarding the ash, is as follows: 84 parts of carbon, 5 parts
of hydrogen, 1 part of nitrogen, 8 parts of oxygen, 2 of sulphur.
The quantity of ash is on an average 5 p.c., but there are coals
which contain a larger quantity, and naturally they are not so
advantageous for use as fuel. The amount of water does not usually
exceed more than 10 p.c. The _anthracites_ form a remarkable
variety of coals, they do not give any volatile products, or but
a very small amount, as they contain but little hydrogen compared
to oxygen. In the average composition of coal we saw that for 5
parts of hydrogen there were 8 parts of oxygen; therefore 4 parts
by weight of hydrogen are capable of forming hydrocarbons, because
1 part of hydrogen is necessary in order to form water with the 8
parts of oxygen. These 4 parts by weight of hydrogen can convert
48 parts of carbon into volatile products, because 1 part of
hydrogen by weight in these substances combines with 12 parts of
carbon. The anthracites differ essentially from this: neglecting
the ash, their average composition is as follows: 94 parts of
carbon, 3 of hydrogen, and 3 of oxygen and nitrogen. According to
the analyses of A. A. Voskresensky, the Grousheffsky anthracite
(Don district) contains: C = 93·8, H = 1·7, ash = 1·5. Therefore
the anthracites contain but little hydrogen capable of combining
with the carbon to form hydrocarbons which burn with a flame.
Anthracites are the oldest forms of coal. The newest and least
transformed coals, which resemble some of the brown varieties, are
the _dry_ coals. They burn with a flame like wood, and leave a
coke having the appearance of lumps of coal, half their component
parts being absorbed by the flame (they contain much hydrogen and
oxygen). The remaining varieties of coal (gas coal, smithy coal,
coking, and anthracite) according to Grüner in all respects form
connecting links between the _dry_ coals and the anthracites.
These coals burn with a very smoky flame, and on being heated
leave _coke_, which bears the same relation to coal that charcoal
does to wood. The quantity and quality of coke vary considerably
with the different sorts of coal from which it is formed. In
practice coals are most often distinguished by the properties
and quantity of the coke which they give. In this particular the
so-called bituminous coals are especially valuable, as even the
slack of this kind gives on dry distillation large spongy masses
of coke. If large pieces of these kinds of coal are subjected
to dry distillation, they, as it were, melt, flow together, and
form caking masses of coke. The best coking coals give 65 p.c. of
dense caking coke. Such coal is very valuable for metallurgical
purposes (_see_ Note 8). Besides coke, the dry distillation of
coal produces gas (_see_ further, illuminating gas, p. 361),
coal-tar (which gives benzene, carbolic acid, naphthalene, tar
for artificial asphalt, &c.) and also an aqueous alkaline liquor
(with wood and lignites the liquid is acid from acetic acid) which
contains ammonium carbonate (_see_ Note 6).
[7] In England in 1850 the output of coal was as much as 48 million
tons, and in latter years it has risen to about 190 millions.
Besides this other countries contribute 300 millions--Russia about
6 millions. The United States of America come next to England with
an output of 160 million tons, then Germany 90 millions; France
produces but little (25 millions), and takes about 5 million tons
from England. Thus the world consumes about 500 million tons of
coal yearly. Besides household purposes, coal is chiefly used as
fuel for steam-engines. As every horse-power (= 75 kilogrammetres
per second) of a steam-engine expends on the average more than
25 kilograms in 24 hours, or in a year (counting stoppages) not
less than 5 tons per horse-power, and there are not less than 40
million horse-power at work in the world, the consumption of coal
for motive-power is at least equal to half the whole production.
For this reason coal serves as a criterion of the industrial
development of a country. About 15 p.c. of coal is used for the
manufacture of cast iron, wrought iron, steel, and articles made
of them.
[8] The principal coal beds of Russia under exploitation are: The Don
basin (150 million poods per annum, 62 poods = 1 ton), the Polish
basin (Dombrovo and others 120 million poods per annum), the Toula
and Riazan beds of the Moscow basin (up to 25 million poods),
the Ural basin (10 million poods), the Caucasian (Kviboul, near
Kutais), the Khirjhis steppes, the smithy coal basin (Gov. of
Tomsk), the Sahaline, &c. The Polish and Moscow basins do not give
any coking coals. The presence of every variety of coal (from
the dry coal near Lisichansk on the Donetz to the anthracites of
the entire south-east basin), the great abundance of excellent
metallurgical coal (coking, _see_ Note 6) in the western part
of the basin, its vast extent (as much as 25,000 sq. versts),
the proximity of the seams to the surface (the shafts are now
from 20 to 100 fathoms deep, and in England and Belgium as deep
as 500 fathoms), the fertility of the soil (black earth), the
proximity of the sea (about 100 versts from the Sea of Azoff) and
of the rivers Donetz, Don, and Dneiper, the most abundant seams
of excellent iron ore (Korsan Mogila, Krivoy Rog, Soulin, &c.,
&c.), copper ore, mercury ore (near Nikitovka, in the Bakhmouth
district of the Ekaterinoslav Gov.), and other ores, the richest
probably in the whole world, the beds of rock-salt (near the
stations of the Stoupka and Brianzovka) the excellent clay of all
kinds (china, fire-clay), gypsum, slate, sandstone, and other
_wealth of the Don coal basin_, give complete assurance of the
fact that with the growth of industrial activity in Russia this
bountiful land of the Cossacks and New Russia will become the
centre of the most extensive productive enterprise, not for the
requirements of Russia alone, but of the whole world, because in
no other place can be found such a concentration of favourable
conditions. The growth of enterprise and knowledge, together with
the extinction of the forests which compels Russia to foster the
production of coal, will help to bring about this desired result.
England with a whole fleet of merchant vessels exports annually
about 25 million tons of coal, the price of which is higher than
on the Donetz (where a pood of worked coal costs less than 5
copecks on the average), where anthracites and semi-anthracites
(like Cardiff or steam coal, which burns without smoke) and coking
and metallurgical coals are able both in quantity and quality to
satisfy the most fastidious requirements of the industry already
existing and rapidly increasing everywhere. The coal mines of
England and Belgium are approaching a state of exhaustion, whilst
in those of the Don basin, only at a depth of 100 fathoms,
1,200,000 million poods of coal lie waiting to be worked.
During the imperfect combustion of volatile substances containing
carbon and hydrogen, the hydrogen and part of the carbon first burn,
and the remainder of the carbon forms soot. Tar, pitch, and similar
substances for this reason burn with a smoky flame. Thus soot is
finely-divided charcoal separated during the imperfect combustion
of the vapours and gases of carbonaceous substances rich in carbon.
Specially-prepared soot (lampblack) is very largely used as a black
paint and a large quantity goes for the manufacture of printers' ink. It
is prepared by burning tar, oil, natural gas, naphtha, &c. The quantity
of organic matter remaining undecomposed in the charcoal depends on the
temperature to which it has been submitted. Charcoal prepared at the
lowest temperature still contains a considerable quantity of hydrogen
and oxygen--even as much as 4 p.c. of hydrogen and 20 p.c. of oxygen.
Such charcoal still preserves the structure of the substance from
which it was obtained. Ordinary charcoal, for instance, in which the
structure of the tree is still visible, is of this kind. On submitting
it to further heating, a fresh quantity of hydrogen with carbon and
oxygen (in the form of gases or volatile matter) may be separated, and
the purest charcoal will be obtained on submitting it to the greatest
heat.[9] If it be required to prepare pure charcoal from soot it is
necessary first to wash it with alcohol and ether in order to remove the
soluble tarry products, and then submit it to a powerful heat to drive
off the impurities containing hydrogen and oxygen. Charcoal however when
completely purified does not change in appearance. Its porosity,[10]
bad conducting power for heat, capability of absorbing the luminous
rays (hence its blackness and opacity), and many other qualities, are
familiar from everyday experience.[11] The specific gravity of charcoal
varies from 1·4 to 1·9, and that it floats on water is due to the air
contained in its pores. If charcoal is reduced to a powder and moistened
with spirit, it immediately sinks in water. It is _infusible_ in the
furnace and even at the temperature of the oxyhydrogen flame. In the
heat generated by means of a powerful galvanic current charcoal only
softens but does not completely melt, and on cooling it is found to
have undergone a complete change both in properties and appearance,
and is more or less transformed into graphite. The physical stability
of charcoal is without doubt allied to its chemical stability. It
is evidently a substance devoid of energy, for it is insoluble in
all known liquids, and _at an ordinary temperature does not combine
with anything_; it is an inactive substance, like nitrogen.[12] But
these properties of charcoal change with a rise of temperature; thus,
unlike nitrogen, charcoal, at a high temperature, combines directly
with oxygen. This is well known, as charcoal burns in air. Indeed, not
only does oxygen _combine with charcoal at a red heat_, but sulphur,
hydrogen, silicon, and also iron and some other metals[12 bis] do
so at a very high temperature--that is, when the molecules of the
charcoal have reached a state of great instability--whilst at ordinary
temperatures neither oxygen, sulphur, nor metals act on charcoal in
any way. When burning in oxygen, charcoal forms carbonic anhydride,
CO_{2}, whilst in the vapours of sulphur, carbon bisulphide, CS_{2}, is
formed, and wrought iron, when acted on by carbon, becomes cast iron.
At the great heat obtained by passing the galvanic current through
carbon electrodes, charcoal combines with hydrogen, forming acetylene,
C_{2}H_{2}. Charcoal does not combine directly with nitrogen, but in the
presence of metals and alkaline oxides, nitrogen is absorbed, forming
a metallic cyanide, as, for instance, potassium cyanide, KCN. From
these few direct combinations which charcoal is capable of entering
into, may be derived those numerous carbonaceous compounds which enter
into the composition of plants and animals, and can be thus obtained
artificially. Certain substances containing oxygen give up a part of
it to charcoal at a relatively low temperature. For instance, nitric
acid when boiled with charcoal gives carbonic anhydride and nitric
peroxide. Sulphuric acid is reduced to sulphurous anhydride when heated
with carbon. When heated to redness charcoal absorbs oxygen from a large
number of the oxides. Even such oxides as those of sodium and potassium,
when heated to redness, yield their oxygen to charcoal although they
do not part with it to hydrogen. Only a few of the oxides, like silica
(oxide of silicon) and lime (calcium oxide) resist the reducing
action of charcoal. Charcoal is capable of changing its physical
condition without undergoing any alteration in its essential chemical
properties--that is, it passes into _isomeric_ or _allotropic forms_.
The two other particular forms in which carbon appears are the _diamond_
and _graphite_. The identity of composition of these with charcoal is
proved by burning an equal quantity of all three separately in oxygen
(at a very high temperature), when each gives the same quantity of
carbonic anhydride--namely, 12 parts of charcoal, diamond, or graphite
in a pure state, yield on burning 44 parts by weight of carbonic
anhydride. The physical properties present a marked contrast; the
densest sorts of charcoal have a density of only 1·9, whilst the density
of graphite is about 2·3, and that of the diamond 3·5. A great many
other properties depend on the density, for instance combustibility.
The lighter charcoal is, the more easily it burns; graphite burns with
considerable difficulty even in oxygen, and the diamond burns only
in oxygen and at a very high temperature. On burning, charcoal, the
diamond, and graphite develop different quantities of heat. One part by
weight of wood charcoal converted by burning into carbonic anhydride
develops 8,080 heat units; dense charcoal separated in gas retorts
develops 8,050 heat units; natural graphite, 7,800 heat units; and the
diamond 7,770. The greater the density the less the heat evolved by the
combustion of the carbon.[13]
[9] As it is difficult to separate from the charcoal the admixture of
ash--that is, the earthy matter contained in the vegetable
substance used for producing charcoal--in order to obtain it
in its purest condition it is necessary to use such organic
substances as do not contain any ash, for example completely
refined or purified crystallised sugar, crystallised tartaric
acid, &c.
[10] The cavities in charcoal are the passages through which those
volatile products formed at the same time as the charcoal have
passed. The degree of porosity of charcoal varies considerably,
and has a technical significance, in different kinds of charcoal.
The most porous charcoal is very light; a cubic metre of wood
charcoal weighs about 200 kilograms. Many of the properties of
charcoal which depend exclusively on its porosity are shared by
many other porous substances, and vary with the density of the
charcoal and depend on the way it was prepared. The property
which charcoal has of absorbing gases, liquids, and many
substances in solution, is a case in point. The densest kind
of charcoal is formed by the action of great heat on sugar and
other fusible substances. The lustrous grey dense coke formed in
gas retorts is also of this character. This dense coke collects
on the internal walls of the retorts subjected to great heat,
and is produced by the vapours and gases separated from the
heated coal in the retorts. In virtue of its density such coke
becomes a good conductor of the galvanic current and approaches
graphite. It is principally used in galvanic batteries. Coke, or
the charcoal remaining from the imperfect combustion of coal and
tarry substances, is also but slightly porous, brilliant, does
not soil or mark paper, is dense, almost devoid of the faculty
of retaining liquids and solids, and does not absorb gases. The
light sorts of charcoal produced from charred wood, on the other
hand, show this absorptive power in a most marked degree. This
property is particularly developed in that very fine and friable
charcoal prepared by heating animal substances such as hides and
bones. _The absorptive power of charcoal_ for gases is similar
to the condensation of gases in spongy platinum. Here evidently
there is a case of the adherence of gases to a solid, precisely
as liquids have the property of adhering to various solids. One
volume of charcoal will absorb the following volumes of gases
(charcoal is capable of absorbing an immense amount of chlorine,
almost equal to its own weight):--
--------------------------------------------------------
Saussure. Favre. Heat emitted
Boxwood Charcoal Cocoanut Charcoal per gram of gas
-------------------------------------------------------
NH_{3} 90 172 vols. 494 units
CO_{2} 35 97 " 158 "
N_{2}O 40 99 " 169 "
HCl 85 165 " 274 "
-------------------------------------------------------
The quantity of gas absorbed by the charcoal increases with the
pressure, and is approximately proportional to it. The quantity
of heat given out by the absorption nearly approaches that set
free on dissolving, or passing into a liquid condition.
Charcoal absorbs not only gases, but a number of other
substances. For instance, alcohol which contains disagreeably
smelling fusel oil, on being mixed with charcoal or filtered
through it, loses most of the fusel oil. The practice of
filtering substances through charcoal in order to get rid of
foreign matters is often applied in chemical and manufacturing
processes. Oils, spirits, various extracts, and vegetable and
other solutions are filtered through charcoal in order to purify
them. The bleaching power of charcoal may be tested by using
various coloured solutions--such as aniline dyes, litmus, &c.
Charcoal, which has absorbed one substance to saturation is still
capable of absorbing certain other substances. Animal charcoal,
produced in a very finely-divided state, especially by heating
bones, makes the best sort for the purposes of absorption. Bone
charcoal is used in large quantities in sugar works for filtering
syrups and all saccharine solutions, in order to purify them,
not only from colouring and odorous matter, but also from the
lime which is mixed with the syrups in order to render them
less unstable during boiling. The absorption of lime by animal
charcoal depends, in all probability, in a great degree on the
mineral component parts of bone charcoal.
[11] Charcoal is a very bad conductor of heat, and therefore forms an
excellent insulator or packing to prevent the transmission of
heat. A charcoal lining is often used in crucibles for heating
many substances, as it does not melt and resists a far greater
heat than many other substances.
[12] The unalterability of charcoal under the action of atmospheric
agencies, which produce changes in the majority of stony and
metallic substances, is often made use of in practice. For
example, charcoal is frequently strewn in boundary ditches. The
surface of wood is often charred to render it durable in those
places where the soil is damp and wood itself would soon rot. The
chambers (or in some works towers) through which acids pass (for
example, sulphuric and hydrochloric) in order to bring them into
contact with gases or liquids, are filled with charcoal or coke,
because at ordinary temperatures it resists the action of even
the strongest acids.
[12 bis] Maquenne (1892) discovered that carbon is capable of combining
with the alkali metals. A 20 p.c. amalgam of the metals was heated
to a red heat with charcoal powder in a stream of hydrogen. The
compounds so obtained possessed, after the mercury had been driven
off, the compositions BaC_{2}, SrC_{2}, CaC_{2}. All these
compounds react with water forming acetylene, for example:
BaC_{2} + 2H_{2}O = C_{2}H_{2} + Ba(OH)_{2}
Maquenne proposes the barium carbide as a source of acetylene. He
obtained this compound by heating carbonate of barium, magnesium
powder, and retort carbon in a Perreau furnace (BaCO_{3} + 3Mg + C
= 3MgO + BaC_{2}). One hundred grams of BaC_{2} evolve 5,200 to
5,400 c.c. of acetylene, mixed with about 2-3 p.c. of hydrogen.
The relation of acetylene, C_{2}H_{2}, to these metallic carbides
is evident from the fact that these metals (Ca, Sr, Ba) replace 2
atoms of hydrogen, and therefore C_{2}Ba corresponds to
C_{2}H_{2}, so that they may be regarded as metallic derivatives
of acetylene. Moissan (1893) obtained similar carbides directly
from the oxides by subjecting them to the action of the voltaic
arc, in the presence of carbon, for instance, BaO + 3C = CO +
C_{2}Ba, although at a furnace heat carbon has no action on the
oxides CaO, BaO, SrO. Concerning Al_{4}C_{5}, _see_ Chapter XVII.
Note 38.
[13] When subjected to pressure, charcoal loses heat, hence the densest
form stands to the less dense as a solid to a liquid, or as a
compound to an element. From this the conclusion may be drawn
that the molecules of graphite are more complex than those of
charcoal, and those of the diamond still more so. The specific
heat shows the same variation, and as we shall see further on,
the increased complexity of a molecule leads to a diminution of
the specific heat. At ordinary temperatures the specific heat of
charcoal is 0·24, graphite 0·20, the diamond 0·147. For retort
carbon Le Chatelier (1893) found that the product of the sp. heat
and atomic weight varies, between 0° and 250°, according to the
formula: = 1·92 + 0·0077_t_, and between 250° and 1000°, = 3·54 +
0·00246_t_ (_see_ Chapter XIV. Note 4).
By means of intense heat charcoal may be transformed into graphite.
If a charcoal rod 4 mm. in diameter and 5 mm. long be enclosed in an
exhausted receiver and the current from 600 Bunsen's elements, placed
in parallel series of 100, be passed through it, the charcoal becomes
strongly incandescent, partially volatilises, and is deposited in the
form of graphite. If sugar be placed in a charcoal crucible and a
powerful galvanic current passed through it, it is baked into a mass
similar to graphite. If charcoal be mixed with wrought iron and heated,
cast iron is formed, which contains as much as five per cent. of
charcoal. If molten cast iron be suddenly chilled, the carbon remains in
combination with the iron, forming so called white cast iron; but if the
cooling proceeds slowly, the greater part of the carbon separates in the
form of graphite, and if such cast iron (so called grey cast iron) be
dissolved in acid, the carbon remains in the form of graphite. Graphite
is met with in nature, sometimes in the form of large compact masses,
sometimes permeating rocky formations like the schists or slates, and
in fact is met with in those places which, in all probability, have
been subjected to the action of subterranean heat.[14] The graphite in
cast iron, and sometimes also natural graphite, occasionally appears in
a crystalline form in the shape of six-sided plates, but more often it
occurs as a compact amorphous mass having the characteristic properties
of the familiar black-lead pencil.[15]
[14] There are places where anthracite gradually changes into graphite
as the strata sink. I myself had the opportunity of observing
this gradual transformation in the valley of Aosta.
[15] Pencils are made of graphite worked up into a homogeneous mass by
disintegrating, powdering, and cleansing it from earthy
impurities; the best kinds are made of completely homogeneous
graphite sawn up into the requisite sticks. Graphite is found in
many places. In Russia the so-called Aliberoffsky graphite is
particularly renowned; it is found in the Altai mountains near
the Chinese frontier; in many places in Finland and likewise
on the banks of the Little Tungouska, Sidoroff also found a
considerable quantity of graphite. When mixed with clay, graphite
is used for making crucibles and pots for melting metals.
Graphite, like most forms of charcoal, still contains a certain
quantity of hydrogen, oxygen, and ash, so that in its natural
state it does not contain more than 98 _p.c._ of carbon.
In practice, graphite is purified simply by washing it when in
a finely-ground state, by which means the bulk of the earthy
matter may be separated. The following process, proposed by
Brodie, consists in mixing the powdered graphite with 1/14 part
of its weight of potassium chlorate. The mixture is then heated
with twice its weight of strong sulphuric acid until no more
odoriferous gases are emitted; on cooling, the mixture is thrown
into water and washed; the graphite is then dried and heated
to a red heat; after this it shrinks considerably in volume
and forms a very fine powder, which is then washed. By acting
on graphite several times with a mixture of potassium chlorate
and nitric acid heated up to 60°, Brodie transformed it into a
yellow insoluble acid substance which he called graphitic acid,
C_{11}H_{4}O_{5}. The diamond remains unchanged when subjected to
this treatment, whilst amorphous charcoal is completely oxidised.
Availing himself of this possibility of distinguishing graphite
from the diamond or amorphous charcoal, Berthelot showed that
when compounds of carbon and hydrogen are decomposed by heat,
amorphous charcoal is mainly formed, whilst when compounds of
carbon with chlorine, sulphur, and boron are decomposed, graphite
is principally deposited.
The diamond is a crystalline and transparent form of carbon. It is of
rare occurrence in nature, and is found in the alluvial deposits of
the diamond mines of Brazil, India, South Africa, &c. It has also been
found in meteorites.[15 bis] It crystallises in octahedra, dodecahedra,
cubes, and other forms of the regular system.[16] The efforts which
have been made to produce diamonds artificially, although they have not
been fruitless, have not as yet led to the production of large-sized
crystals, because those means by which crystals are generally formed
are inapplicable to carbon. Indeed, carbon in all its forms being
insoluble and infusible does not pass into a liquid condition by means
of which crystallisation could take place. Diamonds have several
times been successfully produced in the shape of minute crystals
having the appearance of a black powder, but when viewed under the
microscope appearing transparent, and possessing that hardness which
is the peculiar characteristic of the diamond. This diamond powder is
deposited on the negative electrode, when a weak galvanic current is
passed through liquid chloride of carbon.[16 bis]
[15 bis] Diamonds are found in a particular dense rock, known by the
name of itacolumite, and are dug out of the _débris_ produced by
the destruction of the itacolumite by water. When the _débris_ is
washed the diamonds remain behind; they are principally found in
Brazil, in the provinces of Rio and Bahia, and at the Cape of Good
Hope. The _débris_ gives the black or amorphous diamond,
carbonado, and the ordinary colourless or yellow translucent
diamond. As the diamond possesses a very marked cleavage, the
first operation consists in splitting it, and then roughly and
finely polishing it with diamond powder. It is very remarkable
that Professors P. A. Latchinoff and Eroféeff found (1887) diamond
powder in a meteoric stone which fell in the Government of Penza,
in the district of Krasnoslobodsk, near the settlement of Novo
Urei (Sept. 10, 1886). Up to that time charcoal and graphite (a
special variety, cliftonite) had been found in meteorites and the
diamond only conjectured to occur therein. The Novo Urei meteorite
was composed of siliceous matter and metallic iron (with nickel)
like many other meteorites.
[16] Diamonds are sometimes found in the shape of small balls, and in
that case it is impossible to cut them because directly the
surface is ground or broken they fall into minute pieces.
Sometimes minute diamond crystals form a dense mass like sugar,
and this is generally reduced to diamond powder and used for
grinding. Some known varieties of the diamond are almost opaque
and of a black colour. Such diamonds are as hard as the ordinary
ones, and are used for polishing diamonds and other precious
stones, and also for rock boring and tunnelling.
[16 bis] Hannay, in 1880, obtained diamonds by heating a mixture of
heavy liquid hydrocarbons (paraffin oils) with magnesium in a
thick iron tube. This investigation, however, was not repeated.
Moissan (Paris, 1893) produced diamonds artificially by means of the
high temperature attained in the electrical furnace[17] by dissolving
carbon in molten cast iron, and allowing the solution with an excess of
carbon, to cool under the powerful pressure exerted by rapidly cooling
the metal.[17 bis] K. Chroustchoff attained the same end by means of
silver, which dissolves carbon to the extent of 6 p.c. at a high
temperature. Rousseau, for the same purpose, heated carbide of calcium
in the electric furnace. There is no doubt that all these investigators
obtained the diamond as a transparent body, which burnt into CO_{2}, and
possessed an exceptional hardness, but only in the form of a fine powder.
[17] The _electrical furnace_ is an invention of recent times, and
gives the possibility of obtaining a temperature of 3,500°, which
is not only not obtainable in ordinary furnaces, but even in the
oxyhydrogen flame, whose temperature does not exceed 2,000°. The
electrical furnace consists of two pieces of lime, laid one on
the other. A cavity is made in the lower piece for the reception
of the substance to be melted between two thick electrodes of
dense carbon. On passing a current of 70 volts and 450 ampères
a temperature of 3,000° is easily obtained. At a temperature of
2,500° (100 ampères and 40 volts) not only do all metals melt,
but even lime and magnesia (when placed in the space between
the carbon electrodes, _i.e._ in the voltaic arc) become soft
and crystallise on cooling. At 3,000° lime becomes very fluid,
metallic calcium partially separates out and a carbon compound,
which remains liquid for a long time. At this temperature oxide
of uranium is reduced to the suboxide and metal, zirconia and
rock crystal fuse and partially volatilise, as also does alumina;
platinum, gold, and even carbon distinctly volatilise; the
majority of the metals form carbides. At such a temperature also
cast iron and carbon give graphite, while according to Rousseau,
between 2,000° and 3,000° the diamond passes into graphite and
conversely graphite into the diamond, so that this is a kind of
reversible reaction.
[17 bis] Moissan first investigated the solution of carbon in molten
metals (and the formation of the carbides) such as magnesium,
aluminium, iron, manganese, chromium, uranium, silver, platinum,
and silicon. At the same time Friedel, owing to the discovery of
the diamond in meteoric iron, admitted that the formation of the
diamond is dependent upon the influence of iron and sulphur. With
this object, that is to obtain the diamond, Friedel caused sulphur
to react upon samples of cast iron rich in carbon, in a closed
vessel at a maximum temperature of 500°, and after dissolving the
sulphide of iron formed, he obtained a small quantity of a black
powder which scratched corundum, i.e. diamond. Moissan's
experiments (1893) were more successful, probably owing to his
having employed the electrical furnace. If iron be saturated with
carbon at a temperature between 1,100° and 3,000°, then at
1,100°-1,200° a mixture of amorphous carbon and graphite is
formed, while at 3,000° graphite alone is obtained in very
beautiful crystals. Thus under these conditions the diamond is not
formed, and it can only be obtained if the high temperature be
aided by powerful pressures. For this purpose Moissan took
advantage of the pressure produced in the passage of a mass of
molten cast iron from a liquid into a solid state. He first melted
150-200 grams of iron in the electrical furnace, and quickly
introduced a cylinder of carbon into the molten iron. He then
removed the crucible with the molten iron from the furnace and
plunged it into a reservoir containing water. After treating with
boiling hydrochloric acid, three varieties of carbon were
obtained: (1) a small amount of graphite (if the cooling be
rapid); (2) carbon of a chestnut colour in very fine twisted
threads, showing that it had been subjected to a very high
pressure (a similar variety was met with in various samples of the
Canon Diabolo), and lastly (3) an inconsiderable quantity of an
exceeding dense mass which was freed from the admixture of the
lighter modifications by treatment with _aqua regia_, sulphuric
and hydrofluoric acids, and from which Moissan, by means of liquid
bromoform (sp. gr. 2·900), succeeded in separating some small
pieces, having a greater density than bromoform, which scratched
the ruby and had the properties of the diamond. Some of these
pieces were black, others were transparent and refracted light
strongly. The dark grey tint of the former resembled that of the
black diamonds (carbonado). Their density was between 3 and 3·5.
The transparent specimens had a greasy appearance and seemed to
be, as it were, surrounded by an envelope of carbon. At 1,050°
they did not burn entirely in a current of air, so that the
imperfectly burnt particles, and a peculiar form of grains of a
light ochre colour, which retained their crystalline form, could
be examined under the microscope. Similar grains also remain after
the imperfect combustion of the ordinary diamond. Moissan obtained
the same results by rapidly cooling in a stream of coal gas a
piece of cast iron, saturated with carbon obtained from sugar and
first heated to 2,000°. In this instance he obtained small
crystals of diamonds. K. Chroustchoff showed that at its boiling
point silver dissolves 6 p.c. of carbon. This silver was rapidly
cooled, so that a crust formed on the surface and prevented the
metal expanding, and so produced a powerful pressure. A portion of
the carbon which separates out under these conditions exhibits the
properties of the diamond.
Judging from the fact that carbon forms a number of gaseous bodies
(carbonic oxide, carbonic anhydride, methane, ethylene, acetylene, &c.)
and volatile substances (for example, many hydrocarbons and their most
simple derivatives), and considering that the atomic weight of carbon, C
= 12, approaches that of nitrogen, N = 14, and that of oxygen, O = 16,
and that the compounds CO (carbonic oxide) and N_{2}C_{2} (cyanogen)
are gases, it may be argued that if carbon formed the molecule C_{2},
like N_{2} and O_{2}, it would be a gas. And as through polymerism or
the combination of like molecules (as O_{2} passes into O_{3} or NO_{2}
into N_{2}O_{4}) the temperatures of ebullition and fusion rise (which
is particularly clearly proved with the hydrocarbons of the C_{n}H_{2n}
series), it ought to be considered that _the molecules of charcoal,
graphite, and the diamond are very complex_, seeing that they are
insoluble, non-volatile, and infusible. The aptitude which the atoms of
carbon show for combining together and forming complex molecules appears
in all carbon compounds. Among the volatile compounds of carbon many are
well known the molecules of which contain C_{5} ... C_{10} ... C_{20}
... C_{30}, &c., in general C_{n} where n may be very large, and in none
of the other elements is this faculty of complexity so developed as in
carbon.[18] Up to the present time there are no grounds for determining
the degree of polymerism of the charcoal, graphite, or diamond
molecules, and it can only be supposed that they contain C_{n} where n
is a large quantity. Charcoal and those complex non-volatile organic
substances which represent the gradual transitions to charcoal[19] and
form the principal solid substances of organisms, contain a store or
accumulation of internal power in the form of the energy binding the
atoms into complex molecules. When charcoal or complex compounds of
carbon burn, the energy of the carbon and oxygen is turned into heat,
and this fact is taken advantage of at every turn for the generation of
heat from fuel.[20]
[18] The existence of a molecule S_{6} is known (up to 600°), and it
must he beld that this accounts for the formation of hydrogen
persulphide, H_{2}S_{5}. Phosphorus appears in the molecule
P_{4} and gives P_{4}H_{2}. When expounding the data on specific
heat we shall have occasion to return to the question of the
complexity of the carbon molecule.
[19] The hydrocarbons poor in hydrogen and containing many atoms of
carbon, like chrysene and carbopetrocene, &c.,
C_{_n_}H_{2(_n_-_m_)}, are solids, and less fusible as _n_ and
_m_ increase. They present a marked approach to the properties
of the diamond. And in proportion to the diminution of the water
in the carbohydrates C_{_n_}H_{2_m_}O_{_m_}--for example in the
humic compounds (Note 5)--the transition of complex organic
substances to charcoal is very evident. That residue resembling
charcoal and graphite which is obtained by the separation (by
means of copper sulphate and sodium chloride) of iron from white
cast-iron containing carbon chemically combined with the iron,
also seems, especially after the researches of G. A. Zaboudsky,
to be a complex substance containing C_{12}H_{6}O_{3}. The
endeavours which have been directed towards determining the
measure of complexity of the molecules of charcoal, graphite, and
the diamond will probably at some period lead to the solution
of this problem and will most likely prove that the various
forms of charcoal, graphite, and the diamond contain molecules
of different and very considerable complexity. The constancy of
the grouping of benzene, C_{6}H_{6}, and the wide diffusion and
facility of formation of the carbohydrates containing C_{6} (for
example, cellulose, C_{6}H_{10}O_{5}, glucose, C_{6}H_{12}O_{6})
give reason for thinking that the group C_{6} is the first and
simplest of those possible to free carbon, and it may be hoped
that some time or other it may be possible to get carbon in this
form. Perhaps in the diamond there may be found such a relation
between the atoms as in the benzene group, and in charcoal such
as in carbohydrates.
[20] When charcoal burns, the complex molecule C_{_n_} is resolved into
the simple molecules _n_CO_{2}, and therefore part of the
heat--probably no small amount--is expended in the destruction of
the complex molecule C_{_n_}. Perhaps by burning the most complex
substances, which are the poorest as regards hydrogen, it may be
possible to form an idea of the work required to split up C_{_n_}
into separate atoms.
No other two elements are capable of combining together in such variety
as carbon and hydrogen. The hydrocarbons of the C_{_n_}H_{2_m_} series
in many cases differ widely from each other, although they have some
properties in common. All hydrocarbons, whether gaseous, liquid or
solid, are combustible substances sparingly soluble or insoluble in
water. The liquefied gaseous hydrocarbons, as well as those which are
liquid at ordinary temperatures, and those solid hydrocarbons which
have been liquefied by fusion, have the appearance and property of oily
liquors, more or less viscid, or fluid.[21] The solid hydrocarbons
more or less resemble wax in their properties, although ordinary oils
and wax generally contain oxygen in addition to carbon and hydrogen,
but in relatively small proportion. There are also many hydrocarbons
which have the appearance of tar--as, for instance, metacinnamene and
gutta-percha. Those liquid hydrocarbons which boil at a high temperature
are like oils, and those which have a low boiling point resemble ether,
whilst the gaseous hydrocarbons in many of their properties are akin
to hydrogen. All this tends to show that in hydrocarbons physically
considered the properties of solid non-volatile charcoal are strongly
modified and hidden, whilst those of the hydrogen predominate. All
hydrocarbons are neutral substances (neither basic nor acid), but under
certain conditions they enter into peculiar reactions. It has been seen
in those hydrogen compounds which have been already considered (water,
nitric acid, ammonia) that the hydrogen in almost all cases enters into
reaction, being displaced by metals. The hydrogen of the hydrocarbons,
it may be said, has no metallic character that is to say, it is not
directly[22] displaced by metals, even by such as sodium and potassium.
On the application of more or less heat all hydrocarbons decompose[23]
forming charcoal and hydrogen. The majority of hydrocarbons do not
combine with the oxygen of the air or oxidise at ordinary temperatures,
but under the action of nitric acid and many other oxidising substances
most of them undergo oxidation, in which either a portion of the
hydrogen and carbon is separated, or the oxygen enters into combination,
or else the elements of hydrogen peroxide enter into combination with
the hydrocarbon.[24] When heated in air, hydrocarbons burn, and,
according to the amount of carbon they contain, their combustion is
attended more or less with a separation of soot--that is, finely divided
charcoal--which imparts great brilliancy to the flame, and on this
account many of them are used for the purposes of illumination--as,
for instance, kerosene, coal gas, oil of turpentine. As hydrocarbons
contain reducing elements (that is, those capable of combining with
oxygen), they often act as reducing agents--as, for instance, when
heated with oxide of copper, they burn, forming carbonic anhydride and
water, and leave metallic copper. Gerhardt proved that all hydrocarbons
contain an even number of hydrogen atoms. Therefore, the general formula
for all hydrocarbons is C_{_n_}H_{2_m_} where _n_ and _m_ are whole
numbers. This fact is known as _the law of even numbers_. Hence, the
simplest possible hydrocarbons ought to be: CH_{2}, CH_{4}, CH_{6} ...
C_{2}H_{2}, C_{2}H_{4}, C_{2}H_{6}, C_{2}H_{8} ... but they do not all
exist, since the quantity of H which can combine with a certain amount
of carbon is limited, as we shall learn directly.
[21] The viscosity, or degree of mobility, of liquids is determined
by their internal friction. It is estimated by passing the
liquids through narrow (capillary) tubes, the mobile liquids
passing through with greater facility and speed than the viscid
ones. The viscosity varies with the temperature and nature of the
liquids, and in the case of solutions changes with the amount of
the substance dissolved, but is not proportional to it. So that,
for example, with alcohol at 20° the viscosity will be 69, and
for a 50 p.c. solution 160, the viscosity of water being taken as
100. The volume of the liquid which passes through by experiment
(Poiseuille) and theory (Stokes) is proportional to the time, the
pressure, and the fourth power of the diameter of the (capillary)
tube, and inversely proportional to the length of the tube;
this renders it possible to form comparative estimates of the
coefficients of internal friction and viscosity.
As the complexity of the molecules of hydrocarbons and their
derivatives increases by the addition of carbon (or CH_{2}), so
does the degree of viscosity also rise. The extensive series
of investigations referring to this subject still await the
necessary generalisation. That connection which (already partly
observed) ought to exist between the viscosity and the other
physical and chemical properties, forces us to conclude that
the magnitude of internal friction plays an important part in
molecular mechanics. In investigating organic compounds and
solutions, similar researches ought to stand foremost. Many
observations have already been made, but not much has yet been
done with them; the bare facts and some mechanical data exist,
but their relation to molecular mechanics has not been cleared up
in the requisite degree. It has already been seen from existing
data that the viscosity at the temperature of the absolute
boiling point becomes as small as in gases.
[22] In a number of hydrocarbons and their derivatives such a
substitution of metals for the hydrogen may be attained by
indirect means. The property shown by acetylene, C_{2}H_{2},
and its analogues, of forming metallic derivatives is in this
respect particularly characteristic. Judging from the fact that
carbon is an acid element (that is, gives an acid anhydride with
oxygen), though comparatively slightly acid (for carbonic acid is
not at all a strong acid and compounds of chlorine and carbon,
even CCl_{4}, are not decomposed by water as is the case with
phosphorus chloride and even silicic chloride and boric chloride,
although they correspond with acids of but little energy),
one might expect to find in the hydrogen of hydrocarbons this
faculty for being substituted by metals. The metallic compounds
which correspond with hydrocarbons are known under the name of
organo-metallic compounds. Such, for instance, is zinc ethyl,
Zn(C_{2}H_{5})_{2}, which corresponds with ethyl hydride or
ethane, C_{2}H_{6}, in which two atoms of hydrogen have been
exchanged for one of zinc.
[23] Gaseous and volatile hydrocarbons decompose when passed through a
heated tube. When hydrocarbons are decomposed by heating, the
primary products are generally other more stable hydrocarbons,
among which are acetylene, C_{2}H_{2}, benzene, C_{6}H_{6},
naphthalene, C_{10}H_{8}, &c.
[24] Wagner (1888) showed that when unsaturated hydrocarbons are shaken
with a weak (1 p.c.) solution of potassium permanganate,
KMnO_{4}, at ordinary temperatures, they form glycols--for
example, C_{2}H_{4} yields C_{2}H_{6}O_{2}.
Some of the hydrocarbons are capable of combination, whilst others
do not show that power. Those which contain less hydrogen belong to
the former category, and those which, for a given quantity of carbon,
contain the maximum amount of hydrogen, belong to the latter. The
composition of those last mentioned is expressed by the general formula
C_{_n_}H_{2_n_ + 2}. These so-called _saturated hydrocarbons_ are
incapable of combination.[25] The hydrocarbons CH_{6}, C_{2}H_{8},
C_{3}H_{10}, &c.... do not exist. Those containing the maximum amount
of hydrogen will be represented by CH_{4} (_n_ = 1, 2_n_ + 2 = 4),
C_{2}H_{6} (_n_ = 2), C_{3}H_{8} (n = 3), C_{4}H_{10}, &c. This may be
termed the _law of limits_. Placing this in juxtaposition with the law
of even numbers, it is easy to perceive that the possible hydrocarbons
can be ranged in series, the terms of which may be expressed by the
general formulæ C_{_n_}H_{2_n_+2}, C_{_n_}H_{2_n_}, C_{_n_}H_{2_n_-2},
&c.... Those hydrocarbons which belong to any one of the series
expressible by a general formula are said to be _homologous_0 with
one another. Thus, the hydrocarbons CH_{4}, C_{2}H_{6}, C_{3}H_{8},
C_{4}H_{10}, &c.... are members of the limiting (saturated) homologous
series C_{_n_}H_{2_n_+2}. That is, the difference between the members
of the series is CH_{2}.[26] Not only the composition but also the
properties of the members of a series tend to classification in one
group. For instance, the members of the series C_{_n_}H_{2_n_+2}
are not capable of forming additive compounds, whilst those of the
series C_{_n_}H_{2_n_} are capable of combining with chlorine,
sulphuric anhydride, &c.; and the members of the C_{_n_}H_{2_n_-6}
group, belonging to the coal tar series, are easily nitrated (give
nitro-compounds, Chapter VI.), and have other properties in common.
The physical properties of the members of a given homologous series
vary in some such manner as this; the boiling point generally rises and
the internal friction increases as _n_ increases[27]--that is, with an
increase in the relative amount of carbon and the atomic weight; the
specific gravity also regularly changes as _n_ becomes greater.[28]
[25] My article on this subject appeared in the Journal of the
St. Petersburg Academy of Sciences in 1861. Up to that time,
although many additive combinations with hydrocarbons and their
derivatives were known, they had not been generalised, and were
even continually quoted as cases of substitution. Thus the
combination of ethylene, C_{2}H_{4}, with chlorine, Cl_{2}, was
often regarded as a formation of the products of the substitution
of C_{2}H_{5}Cl and HCl, which it was supposed were held together
as the water of crystallisation is in salts. Even earlier than
this (1857, _Journal of the Petroffsky Academy_) I considered
similar cases as true compounds. In general, according to the
law of limits, an unsaturated hydrocarbon, or its derivative, on
combining with _r_X_{2}, gives a substance which is saturated or
else approaching the limit. The investigations of Frankland with
many organo-metallic compounds clearly showed the limit in the
case of metallic compounds, which we shall constantly refer to
later on.
[26] The conception of homology has been applied by Gerhardt to all
organic compounds in his classical work, 'Traité de Chimie
Organique,' finished in 1855 (4 vols.), in which he divided
all organic compounds into _fatty_ and _aromatic_, which is in
principle still adhered to at the present time, although the
latter are more often called benzene derivatives, on account of
the fact that Kekulé, in his beautiful investigations on the
structure of aromatic compounds, showed the presence in them all
of the 'benzene nucleus,' C_{6}H_{6}.
[27] This is always true for hydrocarbons, but for derivatives of the
lower homologues the law is sometimes different; for instance,
in the series of saturated alcohols, C_{_n_}H_{2_n_+1}(OH),
when _n_ = 0, we obtain water, H(OH), which boils at 100°, and
whose specific gravity at 15° = 0·9992; when _n_ = 1, wood
spirit CH_{3}(OH), which boils at 66°, and at 15° has a specific
gravity = 0·7964; when _n_ = 2, ordinary alcohol, C_{2}H_{5}(OH),
boiling at 78°, specific gravity at 15° = 0·7936, and with
further increase of CH_{2} the specific gravity increases. For
the glycols C_{_n_}H_{2_n_}(OH)_{2} the phenomenon of a similar
kind is still more striking; at first the temperature of the
boiling point and the density increase, and then for higher (more
complex) members of the series diminish. The reason for this
phenomenon, it is evident, must be sought for in the influence
and properties of water, and that strong affinity which, acting
between hydrogen and oxygen, determines many of the exceptional
properties of water (Chapter I.).
[28] As, for example, in the saturated series of hydrocarbons
C_{_n_}H_{2_n_+2}, the lowest member (_n_ = 0) must be taken as
hydrogen H_{2}, a gas which (_t.c._ below -190°) is liquefied
with great difficulty, and when in a liquid state has doubtless
a very small density. Where _n_ = 1, 2, 3, the hydrocarbons
CH_{4}, C_{2}H_{6}, C_{3}H_{8} are gases, more and more readily
liquefiable. The temperature of the absolute boiling point for
CH_{4} =-100°, and for ethane C_{2}H_{6}, and in the higher
members it rises. The hydrocarbon C_{4}H_{10}, liquefies at about
0°. C_{5}H_{12} (there are several isomers) boils at from +9°
(Lvoff) to 37°, C_{6}H_{14} from 58° to 78°, &c. The specific
gravities in a liquid state at 15° are:--
C_{5}H_{12} C_{6}H_{14} C_{7}H_{16} C_{10}H_{22} C_{16}H_{34}
0·63 0·66 0·70 0·75 0·85
Many of the hydrocarbons met with in nature are the products of
organisms, and do not belong to the mineral kingdom. A still greater
number are produced artificially. These are formed by what is termed
the combination of residues. For instance, if a mixture of the vapours
of hydrogen sulphide and carbon bisulphide be passed through a tube
in which copper is heated, this latter absorbs the sulphur from both
the compounds, and the liberated carbon and hydrogen combine to form
a hydrocarbon, methane. If carbon be combined with any metal and this
compound MC_{_n_} be treated with an acid HX, then the haloid X will
give a salt with the metal and the residual carbon and hydrogen will
give a hydrocarbon. Thus cast iron which contains a compound of iron
and carbon gives liquid hydrocarbons like naphtha under the action
of acids. If a mixture of bromo-benzene, C_{6}H_{5}Br, and ethyl
bromide, C_{2}H_{5}Br, be heated with metallic sodium, the sodium
combines with the bromine of both compounds, forming sodium bromide,
NaBr. From the first combination the group C_{6}H_{5} remains, and
from the second C_{2}H_{5}. Having an odd number of hydrogen atoms,
they, in virtue of the law of even numbers, cannot exist alone, and
therefore combine together forming the compound C_{6}H_{5}.C_{2}H_{5}
or C_{8}H_{10} (ethylbenzene). Hydrocarbons are also produced by the
breaking up of more complex organic or hydrocarbon compounds, especially
by heating--that is, by dry distillation. For instance, gum-benzoin
contains an acid called benzoic acid, C_{7}H_{6}O_{2}, the vapours
of which, when passed through a heated tube, split up into carbonic
anhydride, CO_{2}, and benzene, C_{6}H_{6}. Carbon and hydrogen only
unite directly in one ratio of combination--namely, to form acetylene,
having the composition C_{2}H_{2}, which, as compared with other
hydrocarbons, exhibits a very great stability at a somewhat high
temperature.[29]
[29] If, at the ordinary temperature (assuming therefore that the
water formed will be in a liquid state) a gram molecule (26
grams) of acetylene, C_{2}H_{2}, be burnt, 310 thousand calories
will be emitted (Thomsen), and as 12 grams of charcoal produce 97
thousand calories, and 2 grams of hydrogen 69 thousand calories,
it follows that, if the hydrogen and carbon of the acetylene were
burnt there would be only 2 × 97 + 69, or 263 thousand calories
produced. It is evident, then, that acetylene in its formation
absorbs 310-263, or 47 thousand calories.
For considerations relative to the combustion of carbon
compounds, we will first enumerate the quantity of heat separated
by the combustion of definite chemical carbon compounds, and then
give a few figures bearing on the kinds of fuel used in practice.
For molecular quantities in perfect combustion the following
amounts of heat are given out (when gaseous carbonic anhydride
and liquid water are formed), according to Thomsen's data (1)
for gaseous C_{_n_}H_{2_n_ + 2}: 52·8 + 158·8_n_ thousand
calories; (2) for C_{_n_}H_{2_n_}: 17·7 + 158·1_n_ thousand
calories; (3) according to Stohmann (1888) for liquid saturated
alcohols, C_{_n_}H_{2_n_ + 2}O: 11·8 + 156·3_n_, and as the
latent heat of evaporation = about 8·2 + 0·6_n_, in a gaseous
state, 20·0 + 156·9_n_; (4) for monobasic saturated liquid
acids, C_{_n_}H_{2_n_}O_{2}:--95·3 + 154·3_n_, and as their
latent heat of evaporation is about 5·0 + 1·2_n_, in a gaseous
form, about--90 + 155_n_; (5) for solid saturated bibasic acids,
C_{_n_}H_{2_n_-2}O_{4}:--253·8 + 152·6_n_, if they are expressed
as C_{_n_}H_{2_n_}C_{2}H_{2}O_{4}, then 51·4 + 152·6_n_; (6)
for benzene and its liquid homologues (still according to
Stohmann) C_{_n_}H_{2_n_-6}:--158·6 + 156·3_n_, and in a gaseous
form about--155 + 157_n_; (7) for the gaseous homologues of
acetylene, C_{_n_}H_{2_n_-2} (according to Thomsen)--5 + 157_n_.
It is evident from the preceding figures that the group CH_{2},
or CH_{3} substituted for H, on burning gives out from 152 to
159 thousand calories. This is less than that given out by C +
H_{2}, which is 97 + 69 or 166 thousand; the reason for this
difference (it would be still greater if carbon were gaseous)
is the amount of heat separated during the formation of CH_{2}.
According to Stohmann, for dextroglucose, C_{6}H_{12}O_{6}, it
is 673·7; for common sugar, C_{12}H_{22}O_{11}, 1325·7; for
cellulose, C_{6}H_{10}O_{5}, 678·0; starch, 677·5; dextrin,
666·2; glycol, C_{2}H_{6}O_{2}, 281·7; glycerine, 397·2, &c.
The heat of combustion of the following solids (determined
by Stohmann) is expressed per unit of weight: naphthalene,
C_{10}H_{8}, 9,621; urea, CN_{2}H_{4}O, 2,465; white of egg,
5,579; dry rye bread, 4,421; wheaten bread, 4,302; tallow, 9,365;
butter, 9,192; linseed oil, 9,323. The most complete collection
of arithmetical data for the heats of combustion will be found
in V. F. Longinin's work, 'Description of the Various Methods
of Determining the Heats of Combustion of Organic Compounds'
(Moscow, 1894).
The number of units of heat given out by _unit weight_ during the
complete combustion and cooling of the following ordinary kinds
of fuel in their usual state of dryness and purity are:--(1) for
wood charcoal, anthracite, semi-anthracite, bituminous coal and
coke, from 7,200 to 8,200; (2) dry, long flaming coals, and the
best brown coals, from 6,200 to 6,800; (3) perfectly dry wood,
3,500; hardly dry, 2,500; (4) perfectly dry peat, best kind,
4,500; compressed and dried, 3,000; (5) petroleum refuse and
similar liquid hydrocarbons, about 11,000; (6) illuminating gas
of the ordinary composition (about 45 vols. H, 40 vols. CH_{4}, 5
vols. CO, and 5 vols. N), about 12,000; (7) producer gas (_see_
next Chapter), containing 2 vols. carbonic anhydride, 30 vols.
carbonic oxide, and 68 vols. nitrogen _for one part by weight of
the whole carbon burnt_, 5,300, and for one part by weight of the
gas, 910, units of heat; and (8) water gas (_see_ next chapter)
containing 4 vols. carbonic anhydride, 8 vols. N_{2}, 24 vols.
carbonic oxide, and 46 vols. H_{2}, for one part by weight of
the carbon consumed in the _generator_ 10,900, and for one part
by weight of the gas, 3,600 units of heat. In these figures,
as in all calorimetric observations, the water produced by the
combustion of the fuel is supposed to be liquid. As regards the
temperature reached by the fuel, it is important to remark that
for solid fuel it is indispensable to admit (to ensure complete
combustion) twice the amount of air required, but liquid, or
pulverised fuel, and especially gaseous fuel, does not require an
excess of air; therefore, a kilogram of charcoal, giving 8,000
units of heat, requires about 24 kilograms of air (3 kilograms
of air per thousand calories) and a kilogram of producer gas
requires only 0·77 kilogram of air (0·85 kilo. of air per 1,000
calories), 1 kilogram of water gas about 4·5 of air (1·25 kilo.
of air per 1,000 calories).
There is one substance known among the saturated hydrocarbons composed
of 1 atom of carbon and 4 atoms of hydrogen; this is a compound
containing the highest percentage of hydrogen (CH_{4} contains 25 per
cent. of hydrogen), and at the same time it is the only hydrocarbon
whose molecule contains but a single atom of carbon. This saturated
hydrocarbon, CH_{4}, is called _marsh gas_ or _methane_. If vegetable
or animal refuse suffers decomposition in a space where the air has not
free access, or no access at all, then the decomposition is accompanied
with the formation of marsh gas, and this either at the ordinary
temperature, or at a comparatively much higher one. On this account
_plants_, when decomposing under water in _marshes_, give out this
gas.[29 bis] It is well known that if the mud in bogs be stirred up,
the act is accompanied with the evolution of a large quantity of gas
bubbles; these may, although slowly, also separate of their own accord.
The gas which is evolved consists principally of marsh gas.[30] If
wood, coal, or many other vegetable or animal substances are decomposed
by the _action of heat_ without access of air--that is, are subjected
to dry distillation--they, in addition to many other gaseous products
of decomposition (carbonic anhydride, hydrogen, and various other
substances), evolve a great deal of methane. Generally the gas which
is used for lighting purposes is obtained by this means and therefore
always contains marsh gas, mixed with dry hydrogen and other vapours and
gases, although it is subsequently purified from many of them.[31]
As the decomposition of the organic matter, which forms coal, is still
going on underground, the evolution of large quantities of marsh gas
frequently occurs in coal-mines.[32] When mixed with air it forms
an explosive mixture, which forms one of the great dangers of coal
mining, as subterranean work has always to be carried on by lamp-light.
This danger is, however, overcome by the use of Humphry Davy's safety
lamp.[33] Sir Humphry Davy observed that on introducing a piece of
wire gauze into a flame, it absorbs so much heat that combustion does
not proceed beyond it (the unburnt gases which pass through it may
be ignited on the other side). In accordance with this, the flame
of the Davy lamp is surrounded with a thick glass (as shown in the
drawing), and has no communication whatever with the explosive mixture
except through a wire gauze which prevents it igniting the mixture
of the marsh-gas issuing from the coal with air. In some districts,
particularly in those where petroleum is found--as, for instance, near
Baku, where a temple of the Indian fire-worshippers was built, and in
Pennsylvania, and other places--marsh gas in abundance issues from the
earth, and it is used, like coal gas, for the purposes of lighting and
warming.[34] Tolerably pure marsh gas[35] may be obtained by heating
a mixture of an acetate with an alkali. Acetic acid, C_{2}H_{4}O_{2},
on being heated is decomposed into marsh gas and carbonic anhydride,
C_{2}H_{4}O_{2} = CH_{4} + CO_{2}.
[29 bis] Manure which decomposes under the action of bacteria gives off
CO_{2} and CH_{4}.
[30] It is easy to collect the gas which is evolved in marshy places
if a glass bottle be inverted in the water and a funnel put into
it (both filled with water); if the mud of the bottom be now
agitated, the bubbles which rise may be easily caught by the
inverted funnel.
[31] [Illustration: FIG. 58.--General view of gas works. _B_, retorts;
_f_, hydraulic main; _H_ and _I_, tar well; _i_, condensers; _L_,
purifiers; _P_, gasholder.]
Illuminating gas is generally prepared by heating gas coal
(_see_ Note 6) in oval cylindrical horizontal cast-iron or clay
retorts. Several such retorts _BB_ (fig. 58) are disposed in the
furnace _A_, and heated together. When the retorts are heated
to a red heat, lumps of coal are thrown into them, and they
are then closed with a closely fitting cover. The illustration
shows the furnace, with five retorts. Coke (_see_ Note 1, dry
distillation) remains in the retorts, and the volatile products
in the form of vapours and gases travel along the pipe _d_,
rising from each retort. These pipes branch above the stove, and
communicate with the receiver _f_ (hydraulic main) placed above
the furnace. Those products of the dry distillation which most
easily pass from the gaseous into the liquid and solid states
collect in the hydraulic main. From the hydraulic main the
vapours and gases travel along the pipe _g_ and the series of
vertical pipes _j_ (which are sometimes cooled by water trickling
over the surface), where the vapours and gases cool from the
contact of the colder surface, and a fresh quantity of vapour
condenses. The condensed liquids pass from the pipes _g_ and _j_
and into the troughs _H_. These troughs always contain liquid
at a constant level (the excess flowing away) so that the gas
cannot escape, and thus they form, as it is termed, a hydraulic
joint. In the state in which it leaves the condensers the gas
consists principally of the following vapours and gases: (1)
vapour of water, (2) ammonium carbonate, (3) liquid hydrocarbons,
(4) hydrogen sulphide, H_{2}S, (5) carbonic anhydride, CO_{2},
(6) carbonic oxide, CO, (7) sulphurous anhydride, SO_{2}, but
a great part of the illuminating gas consists of (8) hydrogen,
(9) marsh gas, (10) olefiant gas, C_{2}H_{4}, and other gaseous
hydrocarbons. The hydrocarbons (3, 9, and 10), the hydrogen,
and carbonic oxide are capable of combustion, and are useful
component parts, but the carbonic anhydride, the hydrogen
sulphide, and sulphurous anhydride, as well as the vapours of
ammonium carbonate, form an injurious admixture, because they
do not burn (CO_{2}, SO_{2}) and lower the temperature and
brilliancy of the flame, or else, although capable of burning
(for example, H_{2}S, CS_{2}, and others), they give out during
combustion sulphurous anhydride which has a disagreeable smell,
is injurious when inhaled, and spoils many surrounding objects.
In order to separate the injurious products, the gas is washed
with water, a cylinder (not shown in the illustration) filled
with coke continually moistened with water serving for this
purpose. The water coming into contact with the gas dissolves
the ammonium carbonate; hydrogen sulphide, carbonic anhydride,
and sulphurous anhydride, being only partly soluble in water,
have to be got rid of by a special means. For this purpose the
gas is passed through moist lime or other alkaline liquid, as
the above-mentioned gases have acid properties and are therefore
retained by the alkali. In the case of lime, calcium carbonate,
sulphite and sulphide, all solid substances, are formed. It is
necessary to renew the purifying material as its absorbing power
decreases. A mixture of lime and sulphate of iron, FeSO_{4},
acts still better, because the latter, with lime, Ca(HO)_{2},
forms ferrous hydroxide, Fe(HO)_{2} and gypsum, CaSO_{4}. The
suboxide (partly turning into oxide) of iron absorbs H_{2}S,
forming FeS and H_{2}O, and the gypsum retains the remainder of
the ammonia, the excess of lime absorbing carbonic anhydride
and sulphuric anhydride. [In English works a native hydrated
ferric hydroxide is used for removing hydrogen sulphide.] This
purification of the gas takes place in the apparatus _L_, where
the gas passes through perforated trays _m_, covered with
sawdust mixed with lime and sulphate of iron. It is necessary
to remark that in the manufacture of gas it is indispensable
to draw off the vapours from the retorts, so that they should
not remain there long (otherwise the hydrocarbons would in a
considerable degree be resolved into charcoal and hydrogen),
and also to avoid a great pressure of gas in the apparatus,
otherwise a quantity of gas would escape at all cracks such as
must inevitably exist in such a complicated arrangement. For
this purpose there are special pumps (exhausters) so regulated
that they only pump off the quantity of gas formed (the pump is
not shown in the illustration). The purified gas passes through
the pipe _n_ into the gasometer (gasholder) _P_, a dome made of
iron plate. The edges of the dome dip into water poured into a
ring-shaped channel _g_, in which the sides of the dome rise and
fall. The gas is collected in this holder, and distributed to its
destination by pipes communicating with the pipe _o_, issuing
from the dome. The pressure of the dome on the gas enables it, on
issuing from a long pipe, to penetrate through the small aperture
of the burner. A hundred kilograms of coal give about 20 to 30
cubic metres of gas, having a density from four to nine times
greater than that of hydrogen. A cubic metre (1,000 litres) of
hydrogen weighs about 87 grams; therefore 100 kilograms of coal
give about 18 kilograms of gas, or about one-sixth of its weight.
Illuminating gas is generally lighter than marsh gas, as it
contains a considerable amount of hydrogen, and is only heavier
than marsh gas when it contains much of the heavier hydrocarbons.
Thus olefiant gas, C_{2}H_{4}, is fourteen times, and the
vapours of benzene thirty-nine times, heavier than hydrogen, and
illuminating gas sometimes contains 15 p.c. of its volume of
them. The brilliancy of the flame of the gas increases with the
quantity of olefiant gas and similar heavy hydrocarbons, as it
then contains more carbon for a given volume and a greater number
of carbon particles are separated. Gas usually contains from 35
to 60 p.c. of its volume of marsh gas, from 30 to 50 p.c. of
hydrogen, from 3 to 5 p.c. of carbonic oxide, from 2 to 10 p.c.
heavy hydrocarbons, and from 3 to 10 p.c. of nitrogen. Wood gives
almost the same sort of gas as coal and almost the same quantity,
but the wood gas contains a great deal of carbonic anhydride,
although on the other hand there is an almost complete absence
of sulphur compounds. Tar, oils, naphtha, and such materials
furnish a large quantity of good illuminating gas. An ordinary
burner of 8 to 12 candle-power burns 5 to 6 cubic feet of coal
gas per hour, but only 1 cubic foot of naphtha gas. One pood (36
lbs. Eng.) of naphtha gives 500 cubic feet of gas--that is, one
kilogram of naphtha produces about one cubic metre of gas. The
formation of combustible gas by heating coal was discovered in
the beginning of the last century, but only put into practice
towards the end by Le-Bon in France and Murdoch in England. In
England, Murdoch, together with the renowned Watt, built the
first gas works in 1805.
[Illustration: FIG. 59.--Blowpipe. Air is blown in at the
trumpet-shaped mouthpiece, and escapes in a fine stream from the
platinum jet placed at the extremity of the side tube.]
[Illustration: FIG. 60.--Davy safety-lamp. [Modern form.]]
In practice illuminating gas is not only used for lighting
(electricity and kerosene are cheaper in Russia), but also as
the motive power for gas engines (_see_ p. 175), which consume
about half a cubic metre per horse-power per hour; gas is also
used in laboratories for heating purposes. When it is necessary
to concentrate the heat, either the ordinary blowpipe (fig. 59)
is applied, placing the end in the flame and blowing through
the mouthpiece; or, in other forms, gas is passed through the
blowpipe; when a large, hot, smokeless flame is required for
heating crucibles or glass-blowing, a foot-blower is used.
High temperatures, which are often required for laboratory and
manufacturing purposes, are most easily attained by the use of
gaseous fuel (illuminating gas, producer gas, and water gas,
which will be treated of in the following chapter), because
complete combustion may be effected without an access of air. It
is evident that in order to obtain high temperatures means must
be taken to diminish the loss of heat by radiation, and to ensure
perfect combustion.
[32] The gas which is set free in coal mines contains a good deal
of nitrogen, some carbonic anhydride, and a large quantity of
marsh gas. The best means of avoiding an explosion consists
in efficient ventilation. It is best to light coal mines with
electric lamps.
[33] The Davy lamp, of which an improved form is represented in the
accompanying figure, is used for lighting coal and other mines
where combustible gas is found. The wick of the lamp is enclosed
in a thick glass cylinder which is firmly held in a metallic
holder. Over this a metallic cylinder and the wire gauze are
placed. The products of combustion pass through the gauze, and
the air enters through the space between the cylinder and the
wire gauze. To ensure greater safety the lamp cannot be opened
without extinguishing the flame.
[34] In Pennsylvania (beyond the Alleghany mountains) many of the
shafts sunk for petroleum only emitted gas, but many useful
applications for it were found and it was conducted in metallic
pipes to works hundreds of miles distant, principally for
metallurgical purposes.
[35] The purest gas is prepared by mixing the liquid substance called
zinc methyl, Zn(CH_{3})_{2}, with water, when the following
reaction occurs:
Zn(CH_{3})_{2} + 2HOH = Zn(HO)_{2} + 2CH_{3}H.
An alkali--for instance, NaHO--gives with acetic acid a salt,
C_{2}H_{3}NaO_{2}, which on decomposition retains carbonic anhydride,
forming a carbonate, Na_{2}CO_{3}, and marsh gas is given off:
C_{2}H_{3}NaO_{2} + NaHO = Na_{2}CO_{3} + CH_{4}
Marsh gas is difficult to liquefy; it is almost insoluble in water,
and is without taste or smell. The most important point in connection
with its chemical reactions is that it does not combine directly with
anything, whilst the other hydrocarbons which contain less hydrogen than
expressed by the formula C_{_n_}H_{2_n_ + 2} are capable of combining
with hydrogen, chlorine, certain acids, &c.