of particular interest, and, according to the above-mentioned
determinations made by myself, Kirpicheff, and Hemilian, and
verified (by two methods) by K. D. Kraevitch and Prof. Ramsay
(London, 1894), they are proper to all gases (even to those which
are easily compressed into a liquid state, such as carbonic and
sulphurous anhydrides). These discrepancies approach the case
of a very high rarefaction of gases, where a gas is near to a
condition of maximum dispersion of its molecules, and perhaps
presents a passage towards the substance termed 'luminiferous
ether' which fills up interplanetary and interstellar space. If
we suppose that gases are rarefiable to a definite limit only,
having attained which they (like solids) do not alter in volume
with a decrease of pressure, then on the one hand the passage of
the atmosphere at its upper limits into a homogeneous ethereal
medium becomes comprehensible, and on the other hand it would be
expected that gases would, in a state of high rarefaction (_i.e._
when small masses of gases occupy large volumes, or when furthest
removed from a liquid state), present positive discrepancies from
Boyle and Mariotte's law. Our present acquaintance with this
province of highly rarefied gases is very limited (because direct
measurements are exceedingly difficult to make, and are hampered
by possible errors of experiment, which may be considerable), and
its further development promises to elucidate much in respect to
natural phenomena. To the three states of matter (solid, liquid,
and gaseous) it is evident a fourth must yet be added, the
ethereal or ultra-gaseous (as Crookes proposed), understanding by
this, matter in its highest possible state of rarefaction.
[26] The law of Gay-Lussac states that all gases in all conditions
present one coefficient of expansion 0·00367; that is, when
heated from 0° to 100° they expand like air; namely, a thousand
volumes of a gas measured at 0° will occupy 1367 volumes at
100°. Regnault, about 1850, showed that Gay-Lussac's law is not
entirely correct, and that different gases, and also one and
the same gas at different pressures, have not quite the same
coefficients of expansion. Thus the expansion of air between 0°
and 100° is 0·367 under the ordinary pressure of one atmosphere,
and at three atmospheres it is 0·371, the expansion of hydrogen
is 0·366, and of carbonic anhydride 0·37. Regnault, however,
did not directly determine the change of volume between 0° and
100°, but measured the variation of tension with the change of
temperature; but since gases do not entirely follow Mariotte's
law, the change of volume cannot be directly judged by the
variation of tension. The investigations carried on by myself and
Kayander, about 1870, showed the variation of volume on heating
from 0° to 100° under a constant pressure. These investigations
confirmed Regnault's conclusion that Gay-Lussac's law is not
entirely correct, and further showed (1) that the expansion
per volume from 0° to 100° under a pressure of one atmosphere,
for air = 0·368, for hydrogen = 0·367, for carbonic anhydride
= 0·373, for hydrogen bromide = 0·386, &c.; (2) that for gases
which are more compressible than should follow from Mariotte's
law the expansion by heat increases with the pressure--for
example, for air at a pressure of three and a half atmospheres,
it equals 0·371, for carbonic anhydride at one atmosphere
it equals 0·373, at three atmospheres 0·389, and at eight
atmospheres 0·413; (3) that for gases which are less compressible
than should follow from Mariotte's law, the expansion by heat
decreases with an increase of pressure--for example, for hydrogen
at one atmosphere 0·367, at eight atmospheres 0·369, for air
at a quarter of an atmosphere 0·370, at one atmosphere 0·368;
and hydrogen like _air_ (and all gases) is less compressed _at
low pressures_ than should follow from Mariotte's law (_see_
Note 25). Hence, hydrogen, starting from zero to the highest
pressures, exhibits a gradually, although only slightly, varying
coefficient of expansion, whilst for air and other gases at the
atmospheric and higher pressures, the coefficient of expansion
increases with the increase of pressure, so long as their
compressibility is greater than should follow from Mariotte's
law. But when at considerable pressures, this kind of discrepancy
passes into the normal (_see_ Note 25), then the coefficient of
expansion of all gases decreases with an increase of pressure,
as is seen from the researches of Amagat. The difference between
the two coefficients of expansion, for a constant pressure
and for a constant volume, is explained by these relations.
Thus, for example, for air at a pressure of one atmosphere the
true coefficient of expansion (the volume varying at constant
pressure) = 0·00368 (according to Mendeléeff and Kayander) and
the variation of tension (at a constant volume, according to
Regnault) = 0·00367.
[27] Permanent gases are those which cannot be liquefied by an increase
of pressure alone. With a rise of temperature, all gases and
vapours become permanent gases. As we shall afterwards learn,
carbonic anhydride becomes a permanent gas at temperatures above
31°, and at lower temperatures it has a maximum tension, and may
be liquefied by pressure alone.
_The liquefaction_ of gases, accomplished by Faraday (_see_
Ammonia, Chapter VI.) and others, in the first half of this
century, showed that a number of substances are capable, like
water, of taking all three physical states, and that there is
no essential difference between vapours and gases, the only
distinction being that the boiling points (or the temperature
at which the tension = 760 mm.) of liquids lie above the
ordinary temperature, and those of liquefied gases below, and
consequently a gas is a superheated vapour, or vapour heated
above the boiling point, or removed from saturation, rarefied,
having a lower tension than that maximum which is proper to a
given temperature and substance. We will here cite the _maximum
tensions_ of certain liquids and gases _at various temperatures_,
because they may be taken advantage of for obtaining constant
temperatures by changing the pressure at which boiling or the
formation of saturated vapours takes place. (I may remark that
the dependence between the tension of the saturated vapours of
various substances and the temperature is very complex, and
usually requires three or four independent constants, which
vary with the nature of the substance, and are found from the
dependence of the tension _p_ on the temperature _t_ given
by experiment; but in 1892 K. D. Kraevitch showed that this
dependence is determined by the properties of a substance, such
as its density, specific heat, and latent heat of evaporation.)
The temperatures (according to the air thermometer) are placed
on the left, and the tension in millimetres of mercury (at 0°)
on the right-hand side of the equations. Carbon bisulphide,
CS_{2}, 0° = 127·9; 10° = 198·5; 20° = 298·1; 30° = 431·6; 40°
= 617·5; 50° = 857·1. Chlorobenzene, C_{6}H_{5}Cl, 70° = 97·9;
80° = 141·8; 90° = 208·4; 100° = 292·8; 110° = 402·6; 120° =
542·8; 130° = 719·0. Aniline, C_{6}H_{7}N, 150° = 283·7; 160°
= 387·0; 170° = 515·6; 180° = 677·2; 185° = 771·5. Methyl
salicylate, C_{8}H_{8}O_{3}, 180° = 294·4; 190° = 330·9; 200° =
432·4; 210° = 557·5; 220° = 710·2; 224° = 779·9. Mercury, Hg,
300° = 246·8; 310° = 304·9; 320° = 373·7; 330° = 454·4; 340°
= 548·6; 350° = 658·0; 359° = 770·9. Sulphur, S, 395° = 300;
423° = 500; 443° = 700; 452° = 800; 459° = 900. These figures
(Ramsay and Young) show the possibility of obtaining constant
temperatures in the vapours of boiling liquids by altering
the pressure. We may add the following boiling points under
a pressure of 760 mm. (according to the air thermometer by
Collendar and Griffiths, 1891): aniline, 184° = 13; naphthalene,
217° = 94; benzophenone, 305° = 82; mercury, 356° = 76;
triphenyl-methane, 356° = 44; sulphur, 444° = 53. And melting
points: tin, 231° = 68; bismuth, 269° = 22; lead, 327° = 69;
and zinc, 417° = 57. These data may be used for obtaining a
constant temperature and for verifying thermometers. The same
object may be attained by the melting points of certain salts,
determined according to the air thermometer by V. Meyer and
Riddle (1893): NaCl, 851°; NaBr, 727°; NaI, 650°; KCl, 760°;
KBr, 715°; KI, 623°; K_{2}CO_{3}, 1045°; Na_{2}CO_{3}, 1098°;
Na_{2}B_{4}O_{7}, 873°; Na_{2}SO_{4}, 843°; K_{2}SO_{4}, 1073°.
The tension of liquefied gases is expressed in atmospheres.
Sulphurous anhydride, SO_{2},-30° = 0·4;-20° = 0·6;-10° = 1; 0°
= 1·5; +10° = 2·3; 20° = 3·2; 30° = 5·3. Ammonia, NH_{3},-40°
= 0·7;-30° = 1·1;-20° = 1·8; -10° = 2·8; 0° = 4·2; +10° = 6·0;
20° = 8·4. Carbonic anhydride, CO_{2},-115° = 0·033; -80° =
1;-70° = 2·1;-60° = 3·9;-50° = 6·8;-40° = 10;-20° = 23; 0° = 35;
+10° = 46; 20° = 58. Nitrous oxide, N_{2}O,-125° = 0·033;-92° =
1;-80° = 1·9;-50° = 7·6; -20° = 23·1; 0° = 36·1; +20° = 55·3.
Ethylene, C_{2}H_{4},-140° = 0·033;-130° = 0·1; -103° = 1;-40°
= 13;-1° = 42. Air,-191° = 1;-158° = 14;-140° = 39. Nitrogen,
N_{2},-203° = 0·085;-193° = 1;-160° = 14;-146° = 32. The methods
of liquefying gases (by pressure and cold) will be described
under ammonia, nitrous oxide, sulphurous anhydride, and in later
footnotes. We will now turn our attention to the fact that the
evaporation of volatile liquids, under various, and especially
under low, pressures, gives an easy means for obtaining _low
temperatures_. Thus liquefied carbonic anhydride, under the
ordinary pressure, reduces the temperature to -80°, and when it
evaporates in a rarefied atmosphere (under an air-pump) to 25 mm.
(= 0·033 atmosphere) the temperature, judging by the above-cited
figures, falls to -115° (Dewar). Even the evaporation of liquids
of common occurrence, under low pressures easily attainable with
an air-pump, may produce low temperatures, which may be again
taken advantage of for obtaining still lower temperatures. Water
boiling in a vacuum becomes cold, and under a pressure of less
than 4·5 mm. it freezes, because its tension at 0° is 4·5 mm.
A sufficiently low temperature may be obtained by forcing fine
streams of air through common ether, or liquid carbon bisulphide,
CS_{2}, or methyl chloride, CH_{3}Cl, and other similar volatile
liquids. In the adjoining table are given, for certain gases, (1)
the number of atmospheres necessary for their liquefaction at
15°, and (2) the boiling points of the resultant liquids under a
pressure of 760 mm.
C_{2}H_{4} N_{2}O CO_{2}, H_{2}S
(1) 42 31 52 10
(2) -103° -92° -80° -74°
AsH_{3}, NH_{3} HCl CH_{3}Cl C_{2}N_{2} SO_{2}
(1) 8 7 25 4 4 3
(2) -58° -38° -35° -24° -21° -10°
[28] Natterer's determinations (1851-1854), together with Amagat's
results (1880-1888), show that the compressibility of hydrogen,
under high pressures, may be expressed by the following figures:--
_p_ = 1 100 1000 2500
_v_ = 1 0·0107 0·0019 0·0013
_pv_ = 1 1·07 1·9 3·25
_s_ = 0·11 10·3 58 85
where _p_ = the pressure in metres of mercury, _v_ = the volume,
if the volume taken under a pressure of 1 metre = 1, and _s_
the weight of a litre of hydrogen at 20° in grams. If hydrogen
followed Mariotte's law, then under a pressure of 2,500 metres,
one litre would contain not 85, but 265 grams. It is evident
from the above figures that the weight of a litre of the gas
approaches a limit as the pressure increases, which is doubtless
the density of the gas when liquefied, and therefore the weight
of a litre of liquid hydrogen will probably be near 100 grams
(density about 0·1, being less than that of all other liquids).
[29] Cagniard de Latour, on heating ether in a closed tube to about
190°, observed that at this temperature the liquid is transformed
into vapour occupying the original volume--that is, having the
same density as the liquid. The further investigations made by
Drion and myself showed that every liquid has such an _absolute
boiling point_, above which it cannot exist as a liquid and
is transformed into a dense gas. In order to grasp the true
signification of this absolute boiling temperature, it must be
remembered that the liquid state is characterised by a cohesion
of its particles which does not exist in vapours and gases. The
cohesion of liquids is expressed in their capillary phenomena
(the breaks in a column of liquid, drop formation, and rise
in capillary tubes, &c.), and the product of the density of a
liquid into the height to which it rises in a capillary tube (of
a definite diameter) may serve as the measure of the magnitude
of cohesion. Thus, in a tube of 1 mm. diameter, water at 15°
rises (the height being corrected for the meniscus) 14·8 mm.,
and ether at _t°_ to a height 5·35-0·028_t°_ mm. The cohesion
of a liquid is lessened by heating, and therefore the capillary
heights are also diminished. It has been shown by experiment that
this decrement is proportional to the temperature, and hence by
the aid of capillary observations we are able to form an idea
that at a certain rise of temperature the cohesion may become =
0. For ether, according to the above formula, this would occur
at 191°. If the cohesion disappear from a liquid it becomes a
gas, for cohesion is the only point of difference between these
two states. A liquid in evaporating and overcoming the force of
cohesion absorbs heat. Therefore, the absolute boiling point was
defined by me (1861) as that temperature at which (_a_) a liquid
cannot exist as a liquid, but forms a gas which cannot pass into
a liquid state under any pressure whatever; (_b_) cohesion = 0;
and (_c_) the latent heat of evaporation = 0.
This definition was but little known until Andrews (1869)
explained the matter from another aspect. Starting from gases,
he discovered that carbonic anhydride cannot be liquefied by any
degree of compression at temperatures above 31°, whilst at lower
temperatures it can be liquefied. He called this temperature the
_critical temperature_. It is evident that it is the same as
the absolute boiling point. We shall afterwards designate it by
_tc_. At low temperatures a gas which is subjected to a pressure
greater than its maximum tension (Note 27) is transformed into
a liquid, which, in evaporating, gives a saturated vapour
possessing this maximum tension; whilst at temperatures above
tc the pressure to which the gas is subjected may increase
indefinitely. However, under these conditions the volume of the
gas does not change indefinitely but approaches a definite limit
(_see_ Note 28)--that is, it resembles in this respect a liquid
or a solid which is altered but little in volume by pressure.
The volume which a liquid or gas occupies at _tc_ is termed the
_critical volume_, and corresponds with the _critical pressure_,
which we will designate by _pc_ and express in atmospheres. It
is evident from what has been said that the discrepancies from
Mariotte and Boyle's law, the absolute boiling point, the density
in liquid and compressed gaseous states, and the properties of
liquids, must all he intimately connected together. We will
consider these relations in one of the following notes. At
present we will supplement the above observations by the values
of _tc_ and _pc_ for certain liquids and gases which have been
investigated in this respect--
+---------------------+------++----------------------+------+
| _tc_ | _pc_ || _tc_ | _pc_ |
+---------------------+------++----------------------+------+
| N_{2} -146° | 33 || H_{2}S +108° | 92 |
| CO -140° | 39 || C_{2}N_{2} +124° | 62 |
| O_{2} -119° | 50 || NH_{3} +131° | 114 |
| CH_{4} -100° | 50 || CH_{3}Cl +141° | 73 |
| NO -93° | 71 || SO_{2} +155° | 79 |
| C_{2}H_{4} +10° | 51 || C_{5}H_{10} +192° | 34 |
| CO_{2} +32° | 77 || C_{4}H_{10}O +193° | 40 |
| N_{2}O +53° | 75 || CHCl_{3} +268° | 55 |
| C_{2}H_{2} +37° | 68 || CS_{2} +278° | 78 |
| HCl +52° | 86 || C_{6}H_{6} +292° | 60 |
| H_{2}O +365° | 200 || C_{6}H_{5}F +287° | 45 |
| CH_{3}OH +240° | 79 || C_{6}H_{5}Cl +360° | 45 |
| C_{2}H_{5}OH +243° | 63 || C_{6}H_{5}Br +397° | 45 |
| CH_{3}COOH +322° | 57 || C_{6}H_{5}I +448° | 45 |
+---------------------+------++----------------------+------+
Young and Guy (1891) showed that _tc_ and _pc_ clearly depend
upon the composition and molecular weight.
[30] I came to this conclusion in 1870 (_Ann. Phys. Chem._ 141, 623).
[31] Pictet, in his researches, effected the direct liquefaction of
many gases which up to that time had not been liquefied. He
employed the apparatus used for the manufacture of ice on a large
scale, employing the vaporisation of liquid sulphurous anhydride,
which may be liquefied by pressure alone. This anhydride is a gas
which is transformed into a liquid at the ordinary temperature
under a pressure of several atmospheres (_see_ Note 27), and
boils at -10° at the ordinary atmospheric pressure. This liquid,
like all others, boils at a lower temperature under a diminished
pressure, and by continually pumping out the gas which comes
off by means of a powerful air-pump its boiling point falls as
low as -75°. Consequently, if on the one hand we force liquid
sulphurous anhydride into a vessel, and on the other hand pump
out the gas from the same vessel by powerful air-pumps, then the
liquefied gas will boil in the vessel, and cause the temperature
in it to fall to -75°. If a second vessel is placed inside this
vessel, then another gas may be easily liquefied in it at the low
temperature produced by the boiling liquid sulphurous anhydride.
Pictet in this manner easily liquefied carbonic anhydride, CO_{2}
(at -60° under a pressure of from four to six atmospheres).
This gas is more refractory to liquefaction than sulphurous
anhydride, but for this reason it gives on evaporating a still
lower temperature than can be attained by the evaporation of
sulphurous anhydride. A temperature of -80° may be obtained by
the evaporation of liquid carbonic anhydride at a pressure of
760 mm., and in an atmosphere rarefied by a powerful pump the
temperature falls to -140°. By employing such low temperatures, it
was possible, with the aid of pressure, to liquefy the majority
of the other gases. It is evident that special pumps which are
capable of rarefying gases are necessary to reduce the pressure
in the chambers in which the sulphurous and carbonic anhydride
boil; and that, in order to re-condense the resultant gases into
liquids, special force pumps are required for pumping the liquid
anhydrides into the refrigerating chamber. Thus, in Pictet's
apparatus (fig. 24), the carbonic anhydride was liquefied by the
aid of the pumps E F, which compressed the gas (at a pressure of
4-6 atmospheres) and forced it into the tube K, vigorously cooled
by being surrounded by boiling liquid sulphurous anhydride, which
was condensed in the tube C by the pump B, and rarefied by the
pump A. The liquefied carbonic anhydride flowed down the tube K
into the tube H, in which it was subjected to a low pressure by
the pump E, and thus gave a very low temperature of about -140°.
The pump E carried off the vapour of the carbonic anhydride, and
conducted it to the pump F, by which it was again liquefied.
The carbonic anhydride thus made an entire circuit--that
is, it passed from a rarefied vapour of small tension and
low temperature into a compressed and cooled gas, which was
transformed into a liquid, which again vaporised and produced a
low temperature.
[Illustration: FIG. 24.--General arrangement of the apparatus
employed by Pictet for liquefying gases.]
Inside the wide inclined tube H, where the carbonic acid
evaporated, was placed a second and narrow tube M containing
hydrogen, which was generated in the vessel L from a mixture of
sodium formate and caustic soda (CHO_{2}Na + NaHO = Na_{2}CO_{3}
+ H_{2}). This mixture gives hydrogen on heating the vessel L.
This vessel and the tube M were made of thick copper, and could
withstand great pressures. They were, moreover, hermetically
connected together and closed up. Thus the hydrogen which was
evolved had no outlet, accumulated in a limited space, and its
pressure increased in proportion to the amount of it evolved.
This pressure was recorded on a metallic manometer R attached to
the end of the tube M. As the hydrogen in this tube was submitted
to a very low temperature and a powerful pressure, all the
necessary conditions were present for its liquefaction. When the
pressure in the tube H became steady--_i.e._ when the temperature
had fallen to -140° and the manometer R indicated a pressure
of 650 atmospheres in the tube M--then this pressure did not
rise with a further evolution of hydrogen in the vessel L. This
served as an indication that the tension of the vapour of the
hydrogen had attained a maximum corresponding with -140°, and that
consequently all the excess of the gas was condensed to a liquid.
Pictet convinced himself of this by opening the cock N, when the
liquid hydrogen rushed out from the orifice. But, on leaving a
space where the pressure was equal to 650 atmospheres, and coming
into contact with air under the ordinary pressure, the liquid or
powerfully compressed hydrogen expanded, began to boil, absorbed
still more heat, and became still colder. In doing so a portion
of the liquid hydrogen, according to Pictet, passed into a solid
state, and did not fall in drops into a vessel placed under the
outlet N, but as pieces of solid matter, which struck against
the sides of the vessel like shot and immediately vaporised.
Thus, although it was impossible to see and keep the liquefied
hydrogen, still it was clear that it passed not only into a
liquid, but also into a solid state. Pictet in his experiments
obtained other gases which had not previously been liquefied,
especially oxygen and nitrogen, in a liquid and solid state.
Pictet supposed that liquid and solid hydrogen has the properties
of a metal, like iron.
[32] At the same time (1879) as Pictet was working on the liquefaction
of gases in Switzerland, Cailletet, in Paris, was occupied on
the same subject, and his results, although not so convincing as
Pictet's, still showed that the majority of gases, previously
unliquefied, were capable of passing into a liquid state.
Cailletet subjected gases to a pressure of several hundred
atmospheres in narrow thick-walled glass tubes (fig. 25); he
then cooled the compressed gas as far as possible by surrounding
it with a freezing mixture; a cock was then rapidly opened
for the outlet of mercury from the tube containing the gas,
which consequently rapidly and vigorously expanded. This rapid
expansion of the gas would produce great cold, just as the
rapid compression of a gas evolves heat and causes a rise in
temperature. This cold was produced at the expense of the gas
itself, for in rapidly expanding its particles were not able to
absorb heat from the walls of the tube, and in cooling a portion
of the expanding gas was transformed into liquid. This was seen
from the formation of cloud-like drops like a fog which rendered
the gas opaque. Thus Cailletet proved the possibility of the
liquefaction of gases, but he did not isolate the liquids. The
method of Cailletet allows the passage of gases into liquids
being observed with greater facility and simplicity than Pictet's
method, which requires a very complicated and expensive apparatus.
[Illustration: FIG. 25.--Cailletet's apparatus for liquefying
gases.]
The methods of Pictet and Cailletet were afterwards improved by
Olszewski, Wroblewski, Dewar, and others. In order to obtain a
still lower temperature they employed, instead of carbonic acid
gas, liquid ethylene or nitrogen and oxygen, whose evaporation
at low pressures produces a much lower temperature (to -200°).
They also improved on the methods of determining such low
temperatures, but the methods were not essentially altered; they
obtained nitrogen and oxygen in a liquid, and nitrogen even in a
solid, state, but no one has yet succeeded in seeing hydrogen in
a liquid form.
The most illustrative and instructive results (because they
gave the possibility of maintaining a very low temperature
and the liquefied gas, even air, for a length of time) were
obtained in recent years by Prof. Dewar in the Royal Institution
of London, which is glorified by the names of Davy, Faraday,
and Tyndall. Dewar, with the aid of powerful pumps, obtained
many kilograms of oxygen and air (the boiling point under the
atmospheric pressure =-190°) in a liquid state and kept them in
this state for a length of time by means of open glass vessels
with double walls, having a vacuum between them, which prevented
the rapid transference of heat, and so gave the possibility of
maintaining very low temperatures inside the vessel for a long
period of time. The liquefied oxygen or air can be poured from
one vessel into another and used for any investigations. Thus
in June 1894, Prof. Dewar showed that at the low temperature
produced by liquid oxygen many substances become phosphorescent
(become self-luminous; for instance, oxygen on passing into a
vacuum) and fluoresce (emit light after being illuminated; for
instance, paraffin, glue, &c.) much more powerfully than at the
ordinary temperature; also that solids then greatly alter in
their mechanical properties, &c. I had the opportunity (1894)
at Prof. Dewar's of seeing many such experiments in which open
vessels containing pounds of liquid oxygen were employed, and
in following the progress made in researches conducted at low
temperatures, it is my firm impression that the study of many
phenomena at low temperatures should widen the horizon of natural
science as much as the investigation of phenomena made at the
highest temperatures attained in the voltaic arc.
[33] The investigations of S. Wroblewski in Cracow give reason to
believe that Pictet could not have obtained liquid hydrogen in
the interior of his apparatus, and that if he did obtain it,
it could only have been at the moment of its outrush due to
the fall in temperature following its sudden expansion. Pictet
calculated that he obtained a temperature of -140°, but in reality
it hardly fell below -120°, judging from the latest data for
the vaporisation of carbonic anhydride under low pressure. The
difference lies in the method of determining low temperatures.
Judging from other properties of hydrogen (_see_ Note 34), one
would think that its absolute boiling point lies far below -120°,
and even -140° (according to the calculation of Sarrau, on the
basis of its compressibility, at -174°). But even at -200° (if the
methods of determining such low temperatures be correct) hydrogen
does not give a liquid even under a pressure of several hundred
atmospheres. However, on expansion a fog is formed and a liquid
state attained, but the liquid does not separate.
[34] After the idea of the absolute temperature of ebullition (_tc_,
Note 29) had been worked out (about 1870), and its connection
with the deviations from Mariotte's law had become evident, and
especially after the liquefaction of permanent gases, general
attention was turned to the development of the fundamental
conceptions of the gaseous and liquid states of matter. Some
investigators directed their energies to the further study of
vapours (for instance, Ramsay and Young), gases (Amagat), and
liquids (Zaencheffsky, Nadeschdin, and others), especially to
liquids near _tc_ and _pc_; others (Konovaloff and De Heen)
endeavoured to discover the relation between liquids under
ordinary conditions (removed from _tc_ and _pc_) and gases,
whilst a third class of investigators (van der Waals, Clausius,
and others), starting from the generally-accepted principles
of the mechanical theory of heat and the kinetic theory of
gases, and assuming in gases the existence of those forces which
certainly act in liquids, deduced the connection between the
properties of one and the other. It would be out of place in
an elementary handbook like the present to enunciate the whole
mass of conclusions arrived at by this method, but it is well to
give an idea of the results of van der Waals' considerations,
for they explain the gradual uninterrupted passage from a liquid
into a gaseous state in the simplest manner, and, although the
deduction cannot be considered as complete and decisive (_see_
Note 25), nevertheless it penetrates so deeply into the essence
of the matter that its signification is not only reflected in a
great number of physical investigations, but also in the province
of chemistry, where instances of the passage of substances from
a gaseous to a liquid state are so common, and where the very
processes of dissociation, decomposition, and combination must be
identified with a change of physical state of the participating
substances, which has been elaborated by Gibbs, Lavenig, and
others.
For a _given quantity_ (weight, mass) _of a definite substance_,
its state is expressed by three variables--volume _v_, pressure
(elasticity, tension) _p_, and temperature _t_. Although the
compressibility--[_i.e._, _d(v)_/_d(p)_]--of liquids is small,
still it is clearly expressed, and varies not only with the
nature of liquids but also with their pressure and temperature
(at _tc_ the compressibility of liquids is very considerable).
Although gases, according to Mariotte's law, with small
variations of pressure, are uniformly compressed, nevertheless
the dependence of their volume _v_ on _t_ and _p_ is very
complex. This also applies to the coefficient of expansion [=
_d(v)_/_d(t)_, or _d(p)_/_d(t)_], which also varies with _t_ and
_p_, both for gases (_see_ Note 26), and for liquids (at _tc_ it
is very considerable, and often exceeds that of gases, 0·00367).
Hence, the _equation of condition_ must include three variables,
_v_, _p_, and _t_. For a so-called perfect (ideal) gas, or for
inconsiderable variations of density, the elementary expression
_pv_ = _R_[Greek: a](1 + [Greek: a]_t_), or _pv_ = _R_(273 + _t_)
should be accepted, where _R_ is a constant varying with the mass
and nature of a gas, as expressing this dependence, because it
includes in itself the laws of Gay-Lussac and Mariotte, for at a
constant pressure the volume varies proportionally to
1 + [Greek: a]_t_, and when _t_ is constant the product of _tv_ is
constant. In its simplest form the equation may be expressed thus:
_pv_ = _RT_;
where _T_ denotes what is termed the absolute temperature, or the
ordinary temperature + 273--that is, _T_ = _t_ + 273.
Starting from the supposition of the existence of an attraction
or internal pressure (expressed by _a_) proportional to the
square of the density (or inversely proportional to the square of
the volume), and of the existence of a real volume or diminished
length of path (expressed by _b_) for each gaseous molecule, van
der Waals gives for gases the following more complex equation of
condition:--
(_p_ + _a_/_v_^2)(_v_-_b_) = 1 + 0·00367_t_;
if at 0° under a pressure _p_ = 1 (for example, under the
atmospheric pressure), the volume (for instance, a litre) of
a gas or vapour he taken as 1, and therefore _v_ and _b_ be
expressed by the same units as _p_ and _a_. The deviations
from both the laws of Mariotte and Gay-Lussac are expressed
by the above equation. Thus, for hydrogen _a_ must be taken
as infinitely small, and _b_ = 0·0009, judging by the data
for 1,000 and 2,500 metres pressure (Note 28). For other
permanent gases, for which (Note 28) I showed (about 1870) from
Regnault's and Natterer's data, a decrement of _pv_, followed
by an increment, which was confirmed (about 1880) by fresh
determinations made by Amagat, this phenomena may be expressed
in definite magnitudes of _a_ and _b_ (although van der Waals'
formula is not applicable in the case of very small pressures)
with sufficient accuracy for contemporary requirements. It
is evident that van der Waals' formula can also express the
difference of the coefficients of expansion of gases with a
change of pressure, and according to the methods of determination
(Note 26). Besides this, van der Waals' formula shows that at
temperatures above 273(8_a_/27_b_-1) only one actual volume
(gaseous) is possible, whilst at lower temperatures, by varying
the pressure, three different volumes--liquid, gaseous, and
partly liquid, partly saturated-vaporous--are possible. It is
evident that the above temperature is the absolute boiling
point--that is (_tc_) = 273(8_a_/27_b_-1). It is found under the
condition that all three possible volumes (the three roots of
van der Waals' cubic equation) are then similar and equal (_vc_
= 3_b_). The pressure in this case (_pc_) = _a_/(27_b_^2). These
ratios between the constants _a_ and _b_ and the conditions of
_critical state_--_i.e._ (_tc_) and (_pc_)--give the possibility
of determining the one magnitude from the other. Thus for ether
(Note 29), (_tc_) = 193°, (_tp_) = 40, hence _a_ = 0·0307, _b_
= 0·00533, and (_vc_) = 0·016. That mass of ether which at
a pressure of one atmosphere at 0° occupies one volume--for
instance, a litre--occupies, according to the above-mentioned
condition, this critical volume. And as the density of the vapour
of ether compared with hydrogen = 37, and a litre of hydrogen
at 0° and under the atmospheric pressure weighs 0·0896 gram,
then a litre of ether vapour weighs 3·32 grams; therefore, in a
critical state (at 193° and 40 atmospheres) 3·32 grams occupy
0·016 litre, or 16 c.c.; therefore 1 gram occupies a volume of
about 5 c.c., and the weight of 1 c.c. of ether will then be
0·21. According to the investigations of Ramsay and Young (1887),
the critical volume of ether was approximately such at about the
absolute boiling point, but the compressibility of the liquid is
so great that the slightest change of pressure or temperature
has a considerable effect on the volume. But the investigations
of the above savants gave another indirect demonstration of the
truth of van der Waals' equation. They also found for ether that
the isochords, or the lines of equal volumes (if both _t_ and
_p_ vary), are generally straight lines. Thus the volume of 10
c.c. for 1 gram of ether corresponds with pressures (expressed
in metres of mercury) equal to 0·135_t_-3·3 (for example, at
180° the pressure = 21 metres, and at 280° it = 34·5 metres).
The rectilinear form of the isochord (when _v_ = _a_ constant
quantity) is a direct result of van der Waals' formula.
When, in 1883, I demonstrated that the specific gravity of
liquids decreases in proportion to the rise of temperature
[S_{_t_} = S_{_0_}-K_t_ or S_{_t_} = S_{_0_}(1-K_t_)], or that
the volumes increase in inverse proportion to the binomial
1-K_t_, that is, V_{_t_} = V_{_0_}(1-K_t_)^{-1}, where K is the
modulus of expansion, which varies with the nature of the liquid,
then, in general, not only does a connection arise between
gases and liquids with respect to a change of volume, but also
it would appear possible, by applying van der Waals' formula,
to judge, from the phenomena of the expansion of liquids, as
to their transition into vapour, and to connect together all
the principal properties of liquids, which up to this time had
not been considered to be in direct dependence. Thus Thorpe and
Rücker found that 2(_tc_) + 273 = 1/K, where K is the modulus
of expansion in the above-mentioned formula. For example, the
expansion of ether is expressed with sufficient accuracy from
0° to 100° by the equation S_{_t_} = 0·736/(1-0·00154_t_), or
V_{_t_} = 1/(1-0·00154_t_), where 0·00154 is the modulus of
expansion, and therefore (_tc_) = 188°, or by direct observation
193°. For silicon tetrachloride, SiCl_{4}, the modulus equals
0·00136, from whence (_tc_) = 231°, and by experiment 230°. On
the other hand, D. P. Konovaloff, admitting that the external
pressure _p_ in liquids is insignificant when compared with the
internal (_a_ in van der Waals' formula), and that the work in
the expansion of liquids is proportional to their temperature
(as in gases), directly deduced, from van der Waals' formula,
the above-mentioned formula for the expansion of liquids, V_{t}
= 1/(1-K_t_), and also the magnitude of the latent heat of
evaporation, cohesion, and compressibility under pressure. In
this way van der Waals' formula embraces the gaseous, critical,
and _liquid states_ of substances, and shows the connection
between them. On this account, although van der Waals' formula
cannot be considered as perfectly general and accurate, yet it is
not only very much more exact than _pv_ = _RT_, but it is also
more comprehensive, because it applies both to gases and liquids.
Further research will naturally give a closer proximity to
truth, and will show the connection between composition and the
constants (_a_ and _b_); but a great scientific progress is seen
in this form of the equation of state.
Clausius (in 1880), taking into consideration the variability of
_a_, in van der Waals' formula, with the temperature, gave the
following equation of condition:--
(_p_ + _a_/(_T_(_v_ + _c_)^2)) (_v_-_b_) = _RT_.
Sarrau applied this formula to Amagat's data for hydrogen, and
found _a_ = 0·0551, _c_ =-0·00043, _b_ = 0·00089, and therefore
calculated its absolute boiling point as -174°, and (_pc_) = 99
atmospheres. But as similar calculations for oxygen (-105°),
nitrogen (-124°), and marsh gas (-76°) gave _tc_ higher than
it really is, the absolute boiling point of hydrogen must lie
below -174°.
Although a substance which passes with great difficulty into a liquid
state by the action of physico-mechanical forces, hydrogen loses its
gaseous state (that is, its elasticity, or the physical energy of its
molecules, or their rapid progressive motion) with comparative ease
under the influence of chemical attraction,[35] which is not only shown
from the fact that hydrogen and oxygen (two permanent gases) form liquid
water, but also from many phenomena of the absorption of hydrogen.
[35] This and a number of similar cases clearly show how great are the
internal chemical forces compared with physical and mechanical
forces.
Hydrogen is vigorously condensed by certain solids; for example, by
charcoal and by spongy platinum. If a piece of freshly ignited charcoal
be introduced into a cylinder full of hydrogen standing in a mercury
bath, then the charcoal absorbs as much as twice its volume of hydrogen.
Spongy platinum condenses still more hydrogen. But _palladium_, a grey
metal which occurs with platinum, absorbs more hydrogen than any other
metal. Graham showed that when heated to a red heat and cooled in an
atmosphere of hydrogen, palladium retains as much as 600 volumes of
hydrogen. When once absorbed it retains the hydrogen at the ordinary
temperature, and only parts with it when heated to a red heat.[36] This
capacity of certain dense metals for the absorption of hydrogen explains
the property of hydrogen of passing through metallic tubes.[37] It is
termed _occlusion_, and presents a similar phenomenon to solution; it is
based on the capacity of metals of forming unstable easily dissociating
compounds[38] with hydrogen, similar to those which salts form with
water.
[36] The property of palladium of absorbing hydrogen, and of increasing
in volume in so doing, may be easily demonstrated by taking a
sheet of palladium varnished on one side, and using it as a
cathode. The hydrogen which is evolved by the action of the
current is retained by the unvarnished surface, as a consequence
of which the sheet curls up. By attaching a pointer (for
instance, a quill) to the end of the sheet this bending effect is
rendered strikingly evident, and on reversing the current (when
oxygen will be evolved and combine with the absorbed hydrogen,
forming water) it may be shown that on losing the hydrogen the
palladium regains its original form.
[37] Deville discovered that iron and platinum become pervious to
hydrogen at a red heat. He speaks of this in the following
terms:--'The permeability of such homogeneous substances as
platinum and iron is quite different from the passage of gases
through such non-compact substances as clay and graphite. The
permeability of metals depends on their expansion, brought
about by heat, and proves that metals and alloys have a certain
porosity.' However, Graham proved that it is only hydrogen which
is capable of passing through the above-named metals in this
manner. Oxygen, nitrogen, ammonia, and many other gases, only
pass through in extremely minute quantities. Graham showed that
at a red heat about 500 c.c. of hydrogen pass per minute through
a surface of one square metre of platinum 1·1 mm. thick, but that
with other gases the amount transmitted is hardly perceptible.
Indiarubber has the same capacity for allowing the transference
of hydrogen through its substance (_see_ Chapter III.), but at
the ordinary temperature one square metre, 0·014 mm. thick,
transmits only 127 c.c. of hydrogen per minute. In the experiment
on the decomposition of water by heat in porous tubes, the clay
tube may be exchanged for a platinum one with advantage. Graham
showed that by placing a platinum tube containing hydrogen under
these conditions, and surrounding it by a tube containing air,
the transference of the hydrogen may be observed by the decrease
of pressure in the platinum tube. In one hour almost all the
hydrogen (97 p.c.) had passed from the tube, without being
replaced by air. It is evident that the occlusion and passage
of hydrogen through metals capable of occluding it are not only
intimately connected together, but are dependent on the capacity
of metals to form compounds of various degrees of stability with
hydrogen--like salts with water.
[38] It appeared on further investigation that palladium gives a
definite compound, Pd_{2}H (_see_ further) with hydrogen; but
what was most instructive was the investigation of sodium
hydride, Na_{2}H, which clearly showed that the origin and
properties of such compounds are in entire accordance with the
conceptions of dissociation.
Since hydrogen is a gas which is difficult to condense, it is
little soluble in water and other liquids. At 0° a hundred
volumes of water dissolve 1·9 volume of hydrogen, and alcohol 6·9
volumes measured at 0° and 760 mm. Molten iron absorbs hydrogen,
but in solidifying, it expels it. The solution of hydrogen by
metals is to a certain degree based on its affinity for metals,
and must be likened to the solution of metals in mercury and to
the formation of alloys. In its chemical properties hydrogen,
as we shall see later, has much of a metallic character. Pictet
(_see_ Note 31) even affirms that liquid hydrogen has metallic
properties. The metallic properties of hydrogen are also evinced
in the fact that it is a good conductor of heat, which is not the
case with other gases (Magnus).
At the ordinary temperature hydrogen very feebly and rarely enters into
chemical reaction. The capacity of gaseous hydrogen for reaction becomes
evident only under a change of circumstances--by compression, heating,
or the action of light, or at the moment of its evolution. However,
under these circumstances it _combines_ directly with only a very few of
the elements. Hydrogen combines directly with oxygen, sulphur, carbon,
potassium, and certain other elements, but it does not combine directly
with either the majority of the metals or with nitrogen, phosphorus, &c.
Compounds of hydrogen with certain elements on which it does not act
directly are, however, known; they are not obtained by a direct method,
but by reactions of decomposition, or of double decomposition, of other
hydrogen compounds. The property of hydrogen of combining with oxygen
at a red heat determines its combustibility. We have already seen that
hydrogen easily takes fire, and that it then burns with a pale--that is,
non-luminous--flame.[39] Hydrogen does not combine with the oxygen of
the atmosphere at the ordinary temperature; but this combination takes
place at a red heat,[40] and is accompanied by the evolution of much
heat. The product of this combination is water--that is, a compound
of oxygen and hydrogen. This is the _synthesis of water_, and we have
already noticed its analysis or decomposition into its component parts.
The synthesis of water may be very easily observed if a cold glass bell
jar be placed over a burning hydrogen flame, and, better still, if the
hydrogen flame be lighted in the tube of a condenser. The water will
condense in drops as it is formed on the walls of the condenser and
trickle down.[41]
[39] If it be desired to obtain a perfectly colourless hydrogen
flame, it must issue from a platinum nozzle, as the glass end of
a gas-conducting tube imparts a yellow tint to the flame, owing
to the presence of sodium in the glass.
[40] Let us imagine that a stream of hydrogen passes along a tube, and
let us mentally divide this stream into several parts,
consecutively passing out from the orifice of the tube. The first
part is lighted--that is, brought to a state of incandescence,
in which state it combines with the oxygen of the atmosphere. A
considerable amount of heat is evolved in the combination. The
heat evolved then, so to say, ignites the second part of hydrogen
coming from the tube, and, therefore, when once ignited, the
hydrogen continues to burn, if there be a continual supply of it,
and if the atmosphere in which it burns be unlimited and contains
oxygen.
[41] The combustibility of hydrogen may be shown by the direct
decomposition of water by sodium. If a pellet of sodium be thrown
into a vessel containing water, it floats on the water and
evolves hydrogen, which may be lighted. The presence of sodium
imparts a yellow tint to the flame. If potassium be taken, the
hydrogen bursts into flame spontaneously, because sufficient heat
is evolved in the reaction to ignite the hydrogen. The flame is
coloured violet by the potassium. If sodium be thrown not on
to water, but on to an acid, it will evolve more heat, and the
hydrogen will then also burst into flame. These experiments must
be carried on with caution, as, sometimes towards the end, a
mass of sodium oxide (Note 8) is produced, and flies about; it
is therefore best to cover the vessel in which the experiment is
carried on.
Light does not aid the combination of hydrogen and oxygen, so that a
mixture of these two gases does not change when exposed to the action
of light; but an electric spark acts just like a flame, and this is
taken advantage of for inflaming a mixture of oxygen and hydrogen, or
detonating gas, inside a vessel, as will be explained in the following
chapters. As hydrogen (and oxygen also) is condensed by spongy platinum,
by which a rise of temperature ensues, and as platinum acts by contact
(Introduction), therefore hydrogen also combines with oxygen, under
the influence of platinum, as Döbereiner showed. If spongy platinum be
thrown into a mixture of hydrogen and oxygen, an explosion takes place.
If a mixture of the gases be passed over spongy platinum, combination
also ensues, and the platinum becomes red-hot.[42]
[42] This property of spongy platinum is made use of in the so-called
hydrogen cigar-lighter. It consists of a glass cylinder or
beaker, inside which there is a small lead stand (which is not
acted on by sulphuric acid), on which a piece of zinc is laid.
This zinc is covered by a bell, which is open at the bottom
and furnished with a cock at the top. Sulphuric acid is poured
into the space between the bell and the sides of the outer
glass cylinder, and will thus compress the gas in the bell. If
the cock of the cylinder be opened the gas will escape by it,
and will be replaced by the acid, which, coming into contact
with the zinc, evolves hydrogen, and it will escape through the
cock. If the cock be closed, then the hydrogen evolved will
increase the pressure of the gas in the bell, and thus again
force the acid into the space between the bell and the walls of
the outer cylinder. Thus the action of the acid on the zinc may
be stopped or started at will by opening or shutting the cock,
and consequently a stream of hydrogen may be always turned on.
Now, if a piece of spongy platinum be placed in this stream, the
hydrogen will take light, because the spongy platinum becomes
hot in condensing the hydrogen and inflames it. The considerable
rise in temperature of the platinum depends, among other things,
on the fact that the hydrogen condensed in its pores comes into
contact with previously absorbed and condensed atmospheric
oxygen, with which hydrogen combines with great facility in this
form. In this manner the hydrogen cigar-lighter gives a stream of
burning hydrogen when the cock is open. In order that it should
work regularly it is necessary that the spongy platinum should be
quite clean, and it is best enveloped in a thin sheet of platinum
foil, which protects it from dust. In any case, after some time
it will be necessary to clean the platinum, which may be easily
done by boiling it in nitric acid, which does not dissolve the
platinum, but clears it of all dirt. This imperfection has given
rise to several other forms, in which an electric spark is made
to pass before the orifice from which the hydrogen escapes. This
is arranged in such a manner that the zinc of a galvanic element
is immersed when the cock is turned, or a small coil giving a
spark is put into circuit on turning the hydrogen on.
Although gaseous hydrogen does not act directly[43] on many substances,
yet in a _nascent state_ reaction often takes place. Thus, for
instance, water on which sodium amalgam is acting contains hydrogen in
a nascent state. The hydrogen is here evolved from a liquid, and at
the first moment of its formation must be in a condensed state.[44]
In this condition it is capable of reacting on substances on which it
does not act in a gaseous state.[44 bis] Reactions of substitution or
displacement of metals by hydrogen at the moment of its formation are
particularly numerous.[45]
[43] Under conditions similar to those in which hydrogen combines
with oxygen it is also capable of combining with chlorine. A
mixture of hydrogen and chlorine explodes on the passage of an
electric spark through it, or on contact with an incandescent
substance, and also in the presence of spongy platinum; but,
besides this, the action of light alone is enough to bring about
the combination of hydrogen and chlorine. If a mixture of equal
volumes of hydrogen and chlorine be exposed to the action of
sunlight, complete combination rapidly ensues, accompanied by a
report. Hydrogen does not combine directly with carbon, either at
the ordinary temperature or by the action of heat and pressure.
But if an electric current be passed through carbon electrodes
at a short distance from each other (as in the electric light
or voltaic arc), so as to form an electric arc in which the
particles of carbon are carried from one pole to the other, then,
in the intense heat to which the carbon is subjected in this
case, it is capable of combining with hydrogen. A gas of peculiar
smell called acetylene, C_{2}H_{2}, is thus formed from carbon
and hydrogen.
[44] There is another explanation of the facility with which hydrogen
reacts in a nascent state. We shall afterwards learn that the
molecule of hydrogen contains two atoms, H_{2}, but there are
elements the molecules of which only contain one atom--for
instance, mercury. Therefore, every reaction of gaseous hydrogen
must be accompanied by the disruption of that bond which exists
between the atoms forming a molecule. At the moment of evolution,
however, it is supposed that free atoms exist, and in this
condition, according to the hypothesis, act energetically. This
hypothesis is not based upon facts, and the idea that hydrogen is
condensed at the moment of its evolution is more natural, and is
in accordance with the fact (Note 12) that compressed hydrogen
displaces palladium and silver (Brunner, Beketoff)--that is, acts
as at the moment of its liberation.
[44 bis] There is a very intimate and evident relation between the
phenomena which take place in the action of spongy platinum and
the phenomena of the action in a nascent state. The combination
of hydrogen with aldehyde may be taken as an example. Aldehyde is
a volatile liquid with an aromatic smell, boiling at 21°, soluble
in water, and absorbing oxygen from the atmosphere, and in this
absorption forming acetic acid--the substance which is found in
ordinary vinegar. If sodium amalgam be thrown into an aqueous
solution of aldehyde, the greater part of the hydrogen evolved
combines with the aldehyde, forming alcohol--a substance also
soluble in water, which forms the principle of all spirituous
liquors, boils at 78°, and contains the same amount of oxygen
and carbon as aldehyde, but more hydrogen. The composition of
aldehyde is C_{2}H_{4}O, that of alcohol C_{2}H_{6}O.
[45] When, for instance, an acid and zinc are added to a salt of
silver, the silver is reduced; but this may be explained as a
reaction of the zinc, and not of the hydrogen at the moment
of its formation. There are, however, examples to which this
explanation is entirely inapplicable; thus, for instance,
hydrogen, at the moment of its liberation easily takes up oxygen
from its compounds with nitrogen if they be in solution, and
converts the nitrogen into its hydrogen-compound. Here the
nitrogen and hydrogen, so to speak, meet at the moment of their
liberation, and in this state combine together.
It is evident from this that the elastic gaseous state of
hydrogen fixes the limit of its energy: prevents it from entering
into those combinations of which it is capable. In the nascent
state we have hydrogen which is not in a gaseous state, and its
action is then much more energetic. At the moment of evolution
that heat, which would be latent in the gaseous hydrogen, is
transmitted to its molecules, and consequently they are in a
state of strain, and can hence act on many substances.
Metals, as we shall afterwards see, are in many cases able to replace
each other; they also, and in some cases still more easily, replace and
are replaced by hydrogen. We have already seen examples of this in the
formation of hydrogen from water, sulphuric acid, &c. In all these
cases the metals sodium, iron, or zinc displace the hydrogen which
occurs in these compounds. Hydrogen may be displaced from many of its
compounds by metals in exactly the same manner as it is displaced from
water; so, for example, hydrochloric acid, which is formed directly
by the combination of hydrogen with chlorine, gives hydrogen by the
action of a great many metals, just as sulphuric acid does. Potassium
and sodium also displace hydrogen from its compounds with nitrogen; it
is only from its compounds with carbon that hydrogen is not displaced
by metals. Hydrogen, in its turn, is able to replace metals; this is
accomplished most easily on heating, and with those metals which do not
themselves displace hydrogen. If hydrogen be passed over the compounds
of many metals with oxygen at a red heat, it takes up the oxygen from
the metals and displaces them just as it is itself displaced by metals.
If hydrogen be passed over the compound of oxygen with copper at a
red heat, then metallic copper and water are obtained--CuO + H_{2} =
H_{2}O + Cu. This kind of double decomposition is called _reduction_
with respect to the metal, which is thus reduced to a metallic state
from its combination with oxygen. But it must be recollected that all
metals do not displace hydrogen from its compound with oxygen, and,
conversely, hydrogen is not able to displace all metals from their
compounds with oxygen; thus it does not displace potassium, calcium, or
aluminium from its compounds with oxygen. If the metals be arranged in
the following series: K, Na, Ca, Al ... Fe, Zn, Hg ... Cu, Pb, Ag, Au,
then the first are able to take up oxygen from water--that is, displace
hydrogen--whilst the last do not act thus, but are, on the contrary,
reduced by hydrogen--that is, have, as is said, a less affinity for
oxygen than hydrogen, whilst potassium, sodium, and calcium have more.
This is also expressed by the amount of heat evolved in the act of
combination with oxygen (_see_ Note 7), and is shown by the fact that
potassium and sodium and other similar metals evolve heat in decomposing
water; but copper, silver, and the like do not do this, because in
combining with oxygen they evolve less heat than hydrogen does, and
therefore it happens that when hydrogen reduces these metals heat is
evolved. Thus, for example, if 16 grams of oxygen combine with copper,
38,000 units of heat are evolved; and when 16 grams of oxygen combine
with hydrogen, forming water, 69,000 units of heat are evolved; whilst
23 grams of sodium, in combining with 16 grams of oxygen, evolve 100,000
units of heat. This example clearly shows that chemical reactions which
proceed directly and unaided evolve heat. Sodium decomposes water and
hydrogen reduces copper, because they are _exothermal_ reactions, or
those which evolve heat; copper does not decompose water, because such
a reaction would be accompanied by an absorption (or secretion) of heat,
or belongs to the class of _endothermal_ reactions in which heat is
absorbed; and such reactions do not generally proceed directly, although
they may take place with the aid of energy (electrical, thermal, &c.)
borrowed from some foreign source.[46]
[46] Several numerical data and reflections bearing on this matter are
enumerated in Notes 7, 9, and 11. It must be observed that the
action of iron or zinc on water is reversible. But the reaction
CuO + H_{2} = Cu + H_{2}O is not reversible; the difference
between the degrees of affinity is very great in this case,
and, therefore, so far as is at present known, no hydrogen is
liberated even in the presence of a large excess of water. It
is to be further remarked, that under the conditions of the
dissociation of water, copper is not oxidised by water, because
the oxide of copper is reduced by free hydrogen. If a definite
amount of a metal and acid be taken and their reaction be carried
on in a closed space, then the evolution of hydrogen will cease,
when its tension equals that at which compressed hydrogen
displaces the metal. The result depends upon the nature of the
metal and the strength of the solution of acid. Tammann and
Nernst (1892) found that the metals stand in the following order
in respect to this limiting tension of hydrogen:--Na, Mg, Zn, Al,
Cd, Fe, Ni.
[Illustration: FIG. 26.--Apparatus employed by Dumas for determining the
composition of water. Described in text.]
The reduction of metals by hydrogen is taken advantage of for
_determining the exact composition of water by weight_. Copper oxide is
usually chosen for this purpose. It is heated to redness in hydrogen,
and the quantity of water thus formed is determined, when the quantity
of oxygen which occurs in it is found from the loss of weight of the
copper oxide. The copper oxide must be weighed immediately before and
after the experiment. The difference shows the weight of the oxygen
which entered into the composition of the water formed. In this manner
only solids have to be weighed, which is a very great gain in the
accuracy of the results obtained.[47] Dulong and Berzelius (1819) were
the first to determine the composition of water by this method, and
they found that water contains 88·91 of oxygen and 11·09 of hydrogen in
100 parts by weight, or 8·008 parts of oxygen per one part of hydrogen.
Dumas (1842) improved on this method,[48] and found that water contains
12·575 parts of hydrogen per 100 parts of oxygen--that is, 7·990 parts
of oxygen per 1 part of hydrogen--and therefore it is usually accepted
that _water contains eight parts by weight of oxygen to_ one _part by
weight of hydrogen_. By whatever method water be obtained, it will
always present the same composition. Whether it be taken from nature
and purified, or whether it be obtained from hydrogen by oxidation,
or whether it be separated from any of its compounds, or obtained by
some double decomposition--it will in every case contain one part by
weight of hydrogen and eight parts of oxygen. This is because water
is a definite chemical compound. Detonating gas, from which it may be
formed, is a simple mixture of oxygen and hydrogen, although a mixture
of the same composition as water. All the properties of both constituent
gases are preserved in detonating gas. Either one or the other gas may
be added to it without destroying its homogeneity. The fundamental
properties of oxygen and hydrogen are not found in water, and neither
of the gases can be directly combined with it. But they may be evolved
from it. In the formation of water there is an evolution of heat; for
the decomposition of water heat is required. All this is expressed by
the words, _Water is a definite chemical compound of hydrogen with
oxygen_. Taking the symbol of hydrogen, H, as expressing a unit quantity
by weight of this substance, and expressing 16 parts by weight of oxygen
by O, we can formulate all the above statements by the chemical symbol
of water, H_{2}O. As only definite chemical compounds are denoted
by formulæ, having denoted the formula of a compound substance we
express by it the entire series of properties which go to make up our
conception of a definite compound, and at the same time the quantitative
composition of the substance by weight. Further, as we shall afterwards
see, formulæ express the volume of the gases contained in a substance.
Thus the formula of water shows that it contains two volumes of hydrogen
and one volume of oxygen. Besides which, we shall learn that the formula
expresses the density of the vapour of a compound, and on this many
properties of substances depend, and, as we shall learn, determine the
quantities of the bodies entering into reactions. This vapour density we
shall find also determines the quantity of a substance entering into a
reaction. Thus the letters H_{2}O tell the chemist the entire history of
the substance. This is an international language, which endows chemistry
with a simplicity, clearness, stability, and trustworthiness founded on
the investigation of the laws of nature.
[47] This determination may be carried on in an apparatus like that
mentioned in Note 13 of Chapter I.
[48] We will proceed to describe Dumas' method and results. For this
determination pure and dry copper oxide is necessary. Dumas took
a sufficient quantity of copper oxide for the formation of 50
grams of water in each determination. As the oxide of copper
was weighed before and after the experiment, and as the amount
of oxygen contained in water was determined by the difference
between these weights, it was essential that no other substance
besides the oxygen forming the water should be evolved from
the oxide of copper during its ignition in hydrogen. It was
necessary, also, that the hydrogen should be perfectly pure,
and free not only from traces of moisture, but from any other
impurities which might dissolve in the water or combine with the
copper and form some other compound with it. The bulb containing
the oxide of copper (fig. 26), which was heated to redness,
should be quite free from air, as otherwise the oxygen in the
air might, in combining with the hydrogen passing through the
vessel, form water in addition to that formed by the oxygen
of the oxide of copper. The water formed should be entirely
absorbed in order to accurately determine its quantity. The
hydrogen was evolved in the three-necked bottle. The sulphuric
acid, for acting on the zinc, is poured through funnels into
the middle neck. The hydrogen evolved in the Woulfe's bottle
passes through [U] tubes, in which it is purified, to the bulb,
where it comes into contact with the copper oxide, forms water,
and reduces the oxide to metallic copper; the water formed is
condensed in the second bulb, and any passing off is absorbed in
the second set of [U] tubes. This is the general arrangement of
the apparatus. The bulb with the copper oxide is weighed before
and after the experiment. The loss in weight shows the quantity
of oxygen which entered into the composition of the water formed,
the weight of the latter being shown by the gain in weight of
the absorbing apparatus. Knowing the amount of oxygen in the
water formed, we also know the quantity of hydrogen contained in
it, and consequently we determine the composition of water by
weight. This is the essence of the determination. We will now
turn to certain particulars. In one neck of the three-necked
bottle a tube is placed dipping under mercury. This serves as
a safety-valve to prevent the pressure inside the apparatus
becoming too great from the rapid evolution of hydrogen. If the
pressure rose to any considerable extent, the current of gases
and vapours would be very rapid, and, as a consequence, the
hydrogen would not be perfectly purified, or the water entirely
absorbed in the tubes placed for this purpose. In the third neck
of the Woulfe's bottle is a tube conducting the hydrogen to the
purifying apparatus, consisting of eight [U] tubes, destined
for the purification and testing of the hydrogen. The hydrogen,
evolved by zinc and sulphuric acid, is purified by passing it
first through a tube full of pieces of glass moistened with a
solution of lead nitrate next through silver sulphate; the lead
nitrate retains sulphurette hydrogen, and arseniuretted hydrogen
is retained by the tube with silver sulphate. Caustic potash in
the next [U] tube retains any acid which might come over. The
two following tubes are filled with lumps of dry caustic potash
in order to absorb any carbonic anhydride and moisture which the
hydrogen might contain. The next two tubes, to remove the last
traces of moisture, are filled with phosphoric anhydride, mixed
with lumps of pumice-stone. They are immersed in a freezing
mixture. The small [U] tube contains hygroscopic substances,
and is weighed before the experiment: this is in order to know
whether the hydrogen passing through still retains any moisture.
If it does not, then the weight of this tube will not vary during
the whole experiment, but if the hydrogen evolved still retains
moisture, the tube will increase in weight. The copper oxide is
placed in the bulb, which, previous to the experiment, is dried
with the copper oxide for a long period of time. The air is then
exhausted from it, in order to weigh the oxide of copper in a
vacuum and to avoid the need of a correction for weighing in
air. The bulb is made of infusible glass, that it may be able to
withstand a lengthy (20 hours) exposure to a red heat without
changing in form. The weighed bulb is only connected with the
purifying apparatus after the hydrogen has passed through for
a long time, and after experiment has shown that the hydrogen
passing from the purifying apparatus is pure and does not
contain any air. On passing from the condensing bulb the gas and
vapour enter into an apparatus for absorbing the last traces of
moisture. The first [U] tube contains pieces of ignited potash,
the second and third tubes phosphoric anhydride or pumice-stone
moistened with sulphuric acid. The last of the two is employed
for determining whether all the moisture is absorbed, and is
therefore weighed separately. The final tube only serves as a
safety-tube for the whole apparatus, in order that the external
moisture should not penetrate into it. The glass cylinder
contains sulphuric acid, through which the excess of hydrogen
passes; it enables the rate at which the hydrogen is evolved
to be judged, and whether its amount should be decreased or
increased.
When the apparatus is fitted up it must be seen that all its
parts are hermetically tight before commencing the experiment.
When the previously weighed parts are connected together and the
whole apparatus put into communication, then the bulb containing
the copper oxide is heated with a spirit lamp (reduction does
not take place without the aid of heat), and the reduction of
the copper oxide then takes place, and water is formed. When
nearly all the copper oxide is reduced the lamp is removed and
the apparatus allowed to cool, the current of hydrogen being
kept up all the time. When cool, the drawn-out end of the bulb
is fused up, and the hydrogen remaining in it is exhausted, in
order that the copper may be again weighed in a vacuum. The
absorbing apparatus remains full of hydrogen, and would therefore
present a less weight than if it were full of air, as it was
before the experiment, and for this reason, having disconnected
the copper oxide bulb, a current of dry air is passed through it
until the gas passing from the glass cylinder is quite free from
hydrogen. The condensing bulb and the two tubes next to it are
then weighed, in order to determine the quantity of water formed.
Dumas repeated this experiment many times. The average result was
that water contains 1253·3 parts of hydrogen per 10,000 parts
of oxygen. Making a correction for the amount of air contained
in the sulphuric acid employed for producing the hydrogen,
Dumas obtained the average figure 1251·5, between the extremes
1247·2 and 1256·2. This proves that per 1 part of hydrogen water
contains 7·9904 parts of oxygen, with a possible error of not
more than 1/250, or 0·03, in the amount of oxygen per 1 part of
hydrogen.
Erdmann and Marchand, in eight determinations, found that per
10,000 parts of oxygen water contains an average of 1,252 parts
of hydrogen, with a difference of from 1,258·5 to 1,248·7; hence
per 1 part of hydrogen there would be 7·9952 of oxygen, with an
error of at least 0·05.
Keiser (1888), in America by employing palladium hydride, and
by introducing various fresh precautions for obtaining accurate
results, found the composition of water to be 15·95 parts of
oxygen per 2 of hydrogen.
Certain of the latest determinations of the composition of
water, as also those made by Dumas, always give less than 8, and
on the average 7·98, of oxygen per 1 part of hydrogen. However,
not one of these figures is to be entirely depended on, and for
ordinary accuracy it may be considered that O = 16 when H = 1.