NATURE OF SOLUTION. 601 
but may be made to combine in definite proportions by lowering the tempera- 
ture or otherwise varying the conditions. Examples of this are seen in the 
compounds of chlorine, bromine, and iodine with each other. The mixing of 
water with sulphuric acid, as is well known, gives out decreasing quantities of 
heat as dilution goes on, and the point at which no more heat is given out on 
further dilution cannot be determined. We may suppose that the molecules 
of water are associated together in groups, and that when the first portion of 
water is added to the sulphuric acid these groups are completely broken up, 
and each molecule of sulphuric acid attaches to itself a molecule of water to 
form a monohydrate. If another molecule of water be added, the attraction of 
the sulphuric acid, being less, can only partially disintegrate the groups of 
molecules. It could be imagined that the former molecule of water prevents 
the second from getting completely within the sphere of attraction of the mole- 
cule of sulphuric acid. A third molecule would be still less attracted, the 
groups still more incompletely broken up. 
Solution probably depends on the kinetic condition of the bodies, on the 
distances between their molecules, and on the attractions of molecules on each 
other, or rather chemical action between the bodies. 
There are, then, two theories put forward to explain the phenomena of 
solution—one, that stated by BERTHOLLET, may be called the Chemical Theory, 
for it assumes a chemical action between the solvent and the body dissolved ; 
the other may be termed the Physical Theory, since it supposes the molecules 
tu “dissolve” by finding their way into the molecular interspaces of the solvent. 
It is probable that solutions are to be explained by a reference to both theories, 
that some solutions take place in exactly the same way as the diffusion of gases 
into each other, but that others require chemical action to bring them about by 
breaking up congeries of molecules. (The word “solutions” is, of course, not 
intended to include obvious chemical action.) An examination of the effect of 
salts in solution on the solubility of gases might throw some light on the 
subject, if the effect of an equal number of molecules of various salts be studied. 
The solubility of carbonic acid gas in solutions of salts has been studied by 
J. J. Mackenzie (Ann. Chem. Phys. [2] i. 438), who arrived at the following 
results :— 
1. Saline solutions absorb less carbonic acid than an equal volume of water. 
2. The volume of the gas absorbed decreases as the concentration of the 
solution increases. 
3. The coefficient of absorption for solution of potassium chloride lies, like 
its molecular weight, between those of ammonium and sodium chlorides ; 
similarly for strontium, calcium, and barium chlorides. 
As salts, in dissolving in water, generally increase the bulk, a volume of 
saline solution contains less water than an equal volume of pure water. The 
