ON THE THEORY OF SOLUTION. 311 
DISCUSSION ON THE THEORY OF SOLUTION. 
[Ordered by the General Committee to be printed among the Reports. ] 
The present Position of the Hydrate Theory of Solution. 
By SPENCER UMFREVILLE PICKERING, V.A., F.R.S. 
- Ir is but four years since this Section devoted a day to the discussion of 
the nature of solution;' since then, however, the general aspect of the 
question and the position of the: advocates of the two rival theories have 
undergone such a complete change, that in renewing the discussion we 
shall run but little risk of going over the same ground which we then 
trod. At Birmingham, Dr. Tilden opened the discussion by passing in 
review all the well-known and long-known facts which might by any 
possibility throw some light on the nature of solution, and those 
who followed him in the discussion each gave the interpretation 
of these facts which harmonised best with his own views, and, as the facts 
themselves were susceptible of several different interpretations, the not 
surprising restlt followed that each disputant departed holding precisely 
the same opinions which he had brought with him. Since then, however, 
each party has obtained, or thinks that he has obtained, positive evidence 
in favour of his own views ; evidence which, if upheld, must be accepted as 
conclusive, or which must be overthrown before his opponents can claim 
the victory. The supporters of the hydrate theory claim that the curved 
figures representing the properties of solutions of various strengths show 
sudden changes of curvature at certain points, which are the same whai- 
ever be the property examined, which correspond to the composition of 
definite hydrates, and which, therefore, can only be explained by the 
presence of these hydrates in the solutions; while the supporters of the 
physical theory, now identified with the supporters of the osmotic 
pressure theory, claim to have shown that, with weak solutions at 
any rate, the dissolved substance obeys all the laws which are applicable 
to gases, and that, therefore, its molecules must be uninfluenced by, and 
uncombined with, those of the solvent. 
Tn another respect also I may notice that our position to-day differs 
considerably from what it was four years ago; for instead of having to 
argue the matter out amongst ourselves, as we did then, we are now 
favoured with the presence of some of those whose work in this very 
subject has made their names familiar household words with every 
physicist and chemist throughout the scientific world. 
I propose in the first place to give a brief summary of the evidence 
which has lately been adduced in favour of the hydrate theory, and in the 
second place to inquire whether the conclusions drawn from this evidence 
are invalidated by the important facts elucidated by Raoult, van ’t Hoff, 
Arrhenius, and Ostwald. 
Tn one respect the supporters of the hydrate theory start now under a 
distinct advantage, namely, that their most active opponents do not 
altogether deny the existence of hydrates in solution, although it is only 
in the case of strong solutions that they will admit their presence ; in such 
solutions, indeed, it is difficult to see how their presence could possibly 
be denied. The ouly means which we have of proving that a liquid is a 
definite compound is by ascertaining whether its composition remains 
unaltered by its passage through the gaseous or solid condition—by 
1 Report, 1886, p. 444. 
