16 
IOWA ACADEMY OF SCIENCES. 
a. A gas fills uniformly the space to which it is confined. 
A dissolved substance fills uniformly the space (that is the sol- 
vent) to which it is confined. The main difference is that the 
distribution takes place much more slowly in the second case 
than in the first. 
&. A gas exerts a uniform pressure upon limiting surfaces. 
A solution exerts a uniform osmotic pressure. In both cases 
we have, at a constant temperature, an analogous relation; for 
the gas, pressure x volume == a constant, and for the solution, 
osmotic pressure x volume = a constant. 
c. All gases contain the same number of molecules in the 
same space at the same temperature and pressure (Avogadro’s 
Law). All solutions of substances in a given solvent contain the 
same numher of molecules of the dissolved substance in the same 
space at the same temperature and osmotic pressure (Van’t Hoif’s 
Law). Further, to put the analogy in its most general form, 
the pressure exerted by a given amount of substance in the 
state of a gas throughout a given volume is identical with the 
osmotic pressure which the same amount of that substance 
would exert, or the same number of molecules of any otfier 
compound, if dissolved in the given volume of any solvent. 
It is to be noted that, as the familiar ''Laws” of Boyle and 
Charles are not valid for high states of condensation of the 
gases, (in full analogy), the corresponding laws for liquids are 
only valid for relatively dilute solutions. 
The analogy referred to between the gaseous and liquid 
states is, in a word, not merely a superficial one, useful only 
for purposes of illustration, but is of such a profound and far- 
reaching character that all known laws affecting the gases may 
be directly applied, mutatis mutandis, to the liquid state. 
From this analogy the laws of Raoult may be derived, the 
reasoning differing only in form from that previously referred 
to. Aqueous solutions of salts, acids and bases, in general, all 
electrolytic solutions constitute an apparent exception to these 
laws, inasmuch as the indicated molecular weight is much less 
than that calculated from the ordinary chemical formulae. 
Therefore we must conclude, eithor that the laws of Raoult are 
not universally valid and that the theory of Van’t Hoff is 
incomplete, or else that, in the ease of electrolytic solutions, 
the ordinary chemical molecules are split up into two or more 
parts. The latter hypothesis is that adopted by Arrhenius and 
defended and adopted with such fruitful results under the name 
