ON THE THEORY OF SOLUTION. 315 
case warrant such assumptions, or that the constancy and regularity of 
the results are so rigorous as to justify the conclusion that the solvent 
has no action on the dissolved substance, and that there are no irregu- 
larities such as would be caused by the presence of hydrates. 
According to the osmotic pressure theory, the dissolved matter, so 
long, at any rate, as it is not present in greater quantity than it would be 
in the same volume of its gas, if it were gasified under normal conditions, 
is really in the gaseous condition, and obeys all those laws which apply to 
gases. According to the hydrate theory this will be but partially true. 
That the dissolved substance is in a condition comparable with that of a 
gas in so far as the separation of its own particles from each other is 
concerned, must be admitted—indeed, I arrived independently at this 
same conclusion from a study of thermo-chemical data—but inasmuch as 
there is present the solvent, which we believe is not an inactive medium, 
its molecules cannot have the same freedom as if they were truly gaseous, 
and will therefore obey the laws of gases imperfectly only. 
It will be well to confine our attention to but one of those properties 
connected with osmotic pressure, and to select for that purpose the one 
which has been most fully investigated—the lowering of the freezing 
point of a solvent: and the tests which may be applied to ascertain 
whether in producing this lowering the dissolved substance behaves as a 
perfect gas or not, may be grouped under three principal headings :— 
1. Is the molecular depression (i.e. that produced as calculated for 
one molecule dissolved in 100 molecules) constant, independent of the 
nature of the solvent ? 3 
2. Is it independent of the strength of the solution, so long as this 
strength does not exceed the limits (‘ gas’ strength) above mentioned ? 
(Boyle’s law.) 
3. Is it independent of the nature of the dissolved substance ? 
(Avogadro’s law.) 
In the ‘ Phil. Mag.’ 1890, vol. i. p. 495, will be found instances of the 
variation in the molecular depression which may be noticed by altering 
the solvent (see also Table I. above). With water in six different 
solvents it varied between 1°-072 and 0°:003; with sulphuric acid in foar 
different solvents, between 2°15 and 0°°01; with calcium chloride in 
two different solvents, from 2°°773 to 0°01; and with calcium nitrate 
in two solvents, from 2°°5 to 0°°015; while many instances may be 
collected from Raoult’s data showing that the same substance which acts 
normally in one solvent may act abnormally (give only half the usual 
depression) in another. Such variations are so great—from 100 to 
35,600 per cent.—that there can be no doubt but that the solvent is 
not that inert medium which the supporters of the physical theory would 
have it to be, but that it has a very great influence on the results 
obtained. It must be noted, however, that this objection, though applying 
to Raoult’s original views, does not, or, at any rate, may not, apply to 
van ’t Hoff’s theory, for, according to this theory, the nature of the solvent 
has an influence in determining the lowering of the freezing point, W, in 
2 
’ 0 : d 
van ’t Hoff’s equation, dt=—yy_» representing the heat of fusion of the 
solvent. But the lowering is according to this equation independent of 
the nature or the amount of the dissolved substance, so that the two 
following objections will apply to van ’t Hoff’s theory as well as to 
Raoult’s statement. 
