1037 
If by other means, namely by measuring the depression of the 
solidifying point, or the rise in the boiling point of the solutions, 
we try to determine, approximately the value of ¢ according to the 
formula : 
Observed depression S.p. or rise in B.p. 
‘= Moleeular depression S.p. or rise in B.p. x ¢’ 
in which c’ represents the number of gram-mols per 1000 grams 
of water, the values thus found, particularly in the case of concen- 
trated solutions appear to agree very badly with the first named ones. 
Two different causes can be adduced for these divergencies. First, 
the hydrations of the salt molecules and of their ions, owing to 
which a part of the water has been rendered inactive as a solvent. 
Hence, in the last formula a smaller value will be found for c’ 
than it would have been if the salt had been really caleulated on 
1000 grams of solvent. The caleulated value of 7 will thus be greater 
than it would have been without hydration. At high concentrations 
the amount of solvent withdrawn as water of hydration will be 
larger than at low concentrations, so that the influence on 7 will 
be most pronounced in the first case. Also, strongly hydrated salts 
such as MgCl, and CaCl, will exhibit greater differences of 7 than 
the but little hydrated ones such as NaCl and KCl. 
The second cause of the divergencies lies in the relative appli- 
cability of the so-called “ideal gas-laws”. When, according to VAN DER 
Waats, the influence of the factors « and 5 on the gas pressure 
also applies to the osmotic pressure of the solutions, their solidifying and 
boiling points will also be affected thereby. We may compare solutions 
of salts to gases of high molecular weight beeause the mass of 
hydrated particles will be comparatively larger. With concentrations 
of about one gram-mol. per litre we may then expect that the 
factor 4 (volume of the particles) will exert a stronger influence 
than the factor a (proportional to the mutual attraction of the par- 
ticles). The osmotic pressure, therefore also ¢ will then be greater 
than one would expect it to be without those factors. At these large 
concentrations the hydration and the last named circumstance thus 
act on z in the same direction. 
When at smaller concentrations, a becomes predominant, the 
osmotic pressure, hence also 7, will become smaller than would be 
the case according to the ideal gas-laws. Now, as a rule, the question 
is whether « can overcome not only the influence of 5 but also 
that of the hydration of some kind of salt, so that # really 
becomes smaller than would be the case without one of these per- 
turbing factors. 
