A Theory of Solutions. 161 
or may not contain water of crystallization, the coefficient of 
temperature of the electric conductivity*, the phenomena of 
contraction of volume and many others {, show that the process 
of solution is not only a purely physical one, but that there 
exist specific attractions among the particles of the solvent and 
of the dissolved body which will surely cause a combination of 
both. -There would be no special reason why a salt such as 
NaCl should be dissociated, or why the heat of dissociation of 
so many electrolytes should be positive, unless we assumed 
such a combination of the ions with the particles of the solvent. 
We do not suppose that there exist such stable hydrates 
as in the solid state, but we think that the combination is 
of a very mobile kind so that a molecule or an ion of the 
dissolved body wanders from one molecule of the solvent to 
another. There is nobody who to-day would not subscribe 
to these assertions. 
According to the kinetic theory a liquid is in equilibrium 
with its vapour, if in every moment the number of the 
molecules which pass from the vapour to the liquid is the 
same as the number of molecules of the liquid which 
evaporate. The vapour-pressure of a liquid, for example of 
a solvent, is in the first place proportional to the number of 
evaporating particles. There may be dissolved in the solvent 
a non-volatile body, for instance n molecules of the same to 
N molecules of the solvent, and we may suppose that each 
of those n molecules joins with a particles of the solvent. In 
this case an particles of the solvent are prevented from 
evaporating. Since the vapour-pressures of the solvent /% 
and of the solution / are proportional to the numbers of the 
particles which are able to evaporate, we get the equation 
Nt aa ie fo—f _ an 
So N ie 1 
Choosing for dissolved nonelectrolytes a=1, for electrolytes 
with 2 ions a=2, &e., we get the well-known law of Raoult, 
and from that all the other laws of the osmotic theory. 
We have to suppose here that one molecule of sugar, urea, 
&c., always wanders from one molecule of water to another, 
and is combined at every moment with one molecule of the 
solvent ; 1 molecule of NaCl &. ought to be combined with 
2 molecules of water either dissociated or not. 
We are indebted to Prof. Poyntingt{ for those ideas by 
* F’, Kohlrausch, Proc. Roy. Soc. Ixxi. p. 338 (1903). 
+ Hantzsch, Chem. Centrbl. ii. p. 922 (1902); Bogdan, zd¢d. i. p. 71 
(1904) ; and Traube, Wied. Ann. d. Phys. |xii. p. 490 (1897). 
t Poynting, Phil. Mag. [5] xlii. p. 289 (1896), 
Phil. Mag. 8. 6. Vol. 8. No. 44. Aug. 1904. M 
