882 
The reason why the laws which obtain in 
these fields had not been discovered is to be 
found chiefly in the fact that aqueous solu- 
tions had been used, and we kuow to day 
that water dissociates electrolytes, and in 
varying amounts depending upon the dilu- 
tion of the solution. Thus, of all the sol- 
vents which could have been used in study- 
ing these phenomena, no one was so poorly 
adapted to the discovery of any relations 
which might exist as water. 
When Raoult took up the study of the 
lowering of the freezing-point of water by 
dissolved substances, he did not limit 
himself to aqueous solutions, but em- 
ployed solutions in acetic acid, benzene, 
_nitro-benzene, ete. From his work with 
non-aqueous solutions he discovered his 
well-known law: ‘A molecule of any sub- 
stance dissolved in one hundred molecules 
of a liquid lowers the freezing-point of the 
latter a nearly constant amount.’”? The ex- 
perimental data upon which this law is 
based were published in 1884.* 
A little later (1886) Raoult took up the 
study of the lowering of the vapor-ten- 
sion of solvents by dissolved substances. 
Here also he worked with non-aqueous solu- 
tions, and showed that the lowering of the 
yapor-tension is proportional to the ratio 
between the number of molecules of the 
dissolved substance and the total number 
of molecules present. 
The work of Raoult, then, brought out 
this important point, that the lowering of 
the freezing-point of a solvent, as well as the 
lowering of its vapor-tension, depends only 
upon the ratio between the number of parts 
of the dissolved substance and the number 
of parts of the solvent which are present. 
This work of Raoult, however, led toa 
still more important discovery. He found 
that electrolytes, 7. e., those substances which 
in solution conduct the current, give greater 
* Ann. Chim. phys. [6] 2, 66. 
Series, LY. 
Harper’s Science 
SCIENCE. 
[N.S. Vou. XIII. No. 336. 
lowering of the freezing-point of water than 
non-electrolytes. An example will make 
this clear. Hydrochloric acid, sodium hy- 
droxide and sodium chloride in water lower 
its freezing-point nearly twice as much as 
methy] alcohol of the same concentration. 
We recognize in the acid, base and salt 
types of electrolytes, and in methyl] alcohol 
a typical non-electrolyte. Raoult was not 
able to point out the meaning of this im- 
portant discovery. This was left for an- 
other. 
The work of Pfeffer on the osmotic pres- 
sure of solutions led, in the hands of van’t 
Hoff, to the discovery of the relations be- 
tween the osmotic pressure of solutions and 
the gas-pressure of gases. But these re- 
lations hold only for the osmotic pressures 
of non-electrolytes. The electrolytes all 
exert an osmotic pressure which is greater 
than that shown by the non-electrolytes. 
van’t Hoff was not able to explain the 
abnormally large osmotic pressures ex- 
erted by the electrolytes. 
The Swedish physicist, Arrhenius, took 
up this question, and, as we know, furnish- 
ed a satisfactory answer to it. He asked, 
Why is it that electrolytes give abnormally 
large osmotic pressures, abnormally large 
depressions of the freezing-point and vapor- 
tension of solvents? The work of Raoult 
had shown that lowering of freezing-point 
and of vapor-tension depends only upon the 
number of parts present in a given volume 
of the solution. Therefore, when these 
phenomena manifest themselves to an ab- 
normally large degree it means that we 
have more parts present in the solution 
than we suppose. This gave to Arrhenius 
the key to the solution of the problem, and 
the theory of electrolytic dissociation was 
the result. 
This work of Raoult, together with that 
of Pfeffer on osmotic pressure, forms the 
foundation of the new physical chemistry. 
An account of the work of Raoult should 
