observed density of the vapour with that which is calculated
from the hypothesis of such an association to double-molecules,
makes this supposition almost a certainty. In such cases the
molecules in the much denser liquid state must therefore be
considered as double-molecules, either completely so or in a
variable degree depending on the temperature. The given
equation of state cannot hold for such substances. Even though
we assume that a and b are not modified by the formation of
double-molecules, yet RT is modified, and, since it is proportional
to the number of the molecules, is diminished by the combination.
The laws found for normal substances will, therefore,
not hold for such associating substances. Accordingly for
substances for which we have already found an anormal density
of the vapour, we cannot expect the general laws for the liquid
state, which have been treated above, to hold good without
modification, and in many respects such substances will therefore
not follow the law of corresponding states. There are, however,
also substances of which the anormal density of vapour has not
been stated, and which yet cannot be ranged under this law,
e.g. water and alcohols. The most natural thing, of course,
is to ascribe the deviation of these substances, as of the others,
to the fact that the molecules of the liquid are polymerized.
In this case we have to account for the following circumstance,
that whereas for NO2 and acetic acid in the state of saturated
vapour the degree of association increases if the temperature
falls, the reverse must take place for water and alcohols. Such
a difference may be accounted for by the difference in the
quantity of heat released by the polymerization to double-molecules
or larger molecule-complexes. The quantity of heat
given out when two molecules fall together may be calculated
for NO2 and acetic acid from the formula of Gibbs for the
density of vapour, and it proves to be very considerable. With
this the following fact is closely connected. If in the pv diagram,
starting from a point indicating the state of saturated vapour,
a geometrical locus is drawn of the points which have the same
degree of association, this curve, which passes towards isothermals
of higher T if the volume diminishes, requires for the
same change in T a greater diminution of volume than is indicated
by the border-curve. For water and alcohols this geometrical
locus will be found on the other side of the border-curve, and
the polymerization heat will be small, i.e. smaller than the
latent heat. For substances with a small polymerization heat
the degree of association will continually decrease if we move
along the border-curve on the side of the saturated vapour in
the direction towards lower T. With this, it is perfectly compatible
that for such substances the saturated vapour, e.g. under
the pressure of one atmosphere, should show an almost normal
density. Saturated vapour of water at 100° has a density which
seems nearly 4% greater than the theoretical one, an amount
which is greater than can be ascribed to the deviation from
the gas-laws. For the relation between v, T, and x, if x represents
the fraction of the number of double-molecules, the following
formula has been found (“Moleculartheorie,” Zeits. Phys. Chem.,
1890, vol. v):
| log | x(v – b) | = 2 | E1 – E2 | + C, |
| (1 – x)2 | R1T |
from which
| T | ( | dv | ) | = −2 | E1 – E2 | , | |
| v – b | dT | x | R1T |
which may elucidate what precedes.
By far the majority of substances have a value of Tc above the ordinary temperature, and diminution of volume (increase of pressure) is sufficient to condense such gaseous substances into liquids. If Tc is but little above the ordinary temperature, a great increase of pressure is Condensation of substances with low Tc. in general required to effect condensation. Substances for which Tc is much higher than the ordinary temperature T0, e.g. Tc > 53 T0, occur as liquids, even without increase of pressure; that is, at the pressure of one atmosphere. The value 53 is to be considered as only a mean value, because of the inequality of pc. The substances for which Tc is smaller than the ordinary temperature are but few in number. Taking the temperature of melting ice as a limit, these gases are in successive order: CH4, NO, O2, CO, N2 and H2 (the recently discovered gases argon, helium, &c., are left out of account). If these gases are compressed at 0° centigrade they do not show a trace of liquefaction, and therefore they were long known under the name of “permanent gases.” The discovery, however, of the critical temperature carried the conviction that these substances would not be “permanent gases” if they were compressed at much lower T. Hence the problem arose how “low temperatures” were to be brought about. Considered from a general point of view the means to attain this end may be described as follows: we must make use of the above-mentioned circumstance that heat disappears when a substance expands, either with or without performing external work. According as this heat is derived from the substance itself which is to be condensed, or from the substance which is used as a means of cooling, we may divide the methods for condensing the so-called permanent gases into two principal groups.
In order to use a liquid as a cooling bath it must be placed in a vacuum, and it must be possible to keep the pressure of the vapour in that space at a small value. According to the boiling-law, the temperature of the liquid must descend to that at which the maximum tension of the Liquids as means of cooling. vapour is equal to the pressure which reigns on the surface of the liquid. If the vapour, either by means of absorption or by an air-pump, is exhausted from the space, the temperature of the liquid and that of the space itself depend upon the value of the pressure which finally prevails in the space. From a practical point of view the value of T3 may be regarded as the limit to which the temperature falls. It is true that if the air is exhausted to the utmost possible extent, the temperature may fall still lower, but when the substance has become solid, a further diminution of the pressure in the space is of little advantage. At any rate, as a solid body evaporates only on the surface, and solid gases are bad conductors of heat, further cooling will only take place very slowly, and will scarcely neutralize the influx of heat. If the pressure p3 is very small, it is perhaps practically impossible to reach T3; if so, T3 in the following lines will represent the temperature practically attainable. There is thus for every gas a limit below which it is not to be cooled further, at least not in this way. If, however, we can find another gas for which the critical temperature is sufficiently above T3 of the first chosen gas, and if it is converted into a liquid by cooling with the first gas, and then treated in the same way as the first gas, it may in its turn be cooled down to (T3)2. Going on in this way, continually lower temperatures may be attained, and it would be possible to condense all gases, provided the difference of the successive critical temperatures of two gases fulfils certain conditions. If the ratio of the absolute critical temperatures for two gases, which succeed one another in the series, should be sensibly greater than 2, the value of T3 for the first gas is not, or not sufficiently, below the Tc of the second gas. This is the case when one of the gases is nitrogen, on which hydrogen would follow as second gas. Generally, however, we shall take atmospheric air instead of nitrogen. Though this mixture of N2 and O2 will show other critical phenomena than a simple substance, yet we shall continue to speak of a Tc for air, which is given at −140° C., and for which, therefore, Tc amounts to 133° absolute. The lowest T which may be expected for air in a highly rarefied space may be evaluated at 60° absolute—a value which is higher than the Tc for hydrogen. Without new contrivances it would, accordingly, not be possible to reach the critical temperature of H2. The method by which we try to obtain successively lower temperatures by making use of successive gases is called the “cascade method.” It is not self-evident that by sufficiently diminishing the pressure on a liquid it may be cooled to such a degree that the temperature will be lowered to T3, if the initial temperature was equal to Tc, or but little below it, and we can even predict with certainty that this will not be the case for all substances. It is possible, too, that long before the triple point is reached the whole liquid will have evaporated. The most favourable conditions will, of
