592 CYRUS F. TOLMAN 
solution to form some undissociated acid and base (Na Cl + 
HOH Z2=. NaOH +2 HCl). But: sincetthere asisuchwa wemy 
small amount of free dissociated ions of water and the disso- 
ciation constants of sodic and hydrochloric acid are large, this 
action is very slight. However, the salts of the weakest acids 
and bases are affected more by the same amount of dissociated 
water than those of the stronger ones, because, as shown by 
equation (2); K is smaller, therefore a larger amount of the 
undissociated acid, or base, exists in proportion to the ions. 
Water when heated becomes more dissociated, and therefore 
a stronger acid, and this fact is sometimes made use of in the 
analytical laboratory, and is an important factor determining the 
composition of minerals deposited from hot solutions. 
Solutions of gases in pure water—Ilf a gas dissolves in a sub- 
stance with which it is perfectly neutral, that is, if no chemical 
reaction takes place between the gas and the solvent, then the 
amount of the gas dissolved in the liquid at a given tempera- 
ture varies directly with the gaseous pressure, or partial pressure, 
if there be more than one gas. 
Now, if any gas were absolutely neutral to the solvent, and if 
on solution its molecule suffered no change in constitution, we 
should get the same amount of heat absorbed or given off in the 
solution that would be absorbed or radiated by the same 
amount of gas brought to the density of the dissolved gas. 
But upon solution in water it is found that some gases suffer 
a larger heat change than can be explained by any mere physi- 
cal change in volume, while others have the same or nearly the 
same change in temperature, as that calculated upon the 
assumption of simple physical absorption. It has been shown 
that the gases with this large heat change upon solution undergo 
besides physical diffusion, a chemical change, more or less exten- 
sive. Therefore, gases may be arranged in two classes. The 
first class is composed of those gases which have but a slight 
reaction with water, and are sparingly soluble in that menstruum, 
and the other, of those which have large coefficients of solubility 
and which react with water to form definite compounds. 
