PBESIDENTIAD ADDRESS — SECTION B. 85 



Planck, Ostwald, Duhem, Vernst, de Vries, and other physical 

 chemists, chiefly in Germany. The Hterature of the subject is 

 already very large, and is scattered through many scientific 

 journals, some of which have not been accessible to me here in 

 Melbourne ; so that such knowledge as I have of this part of 

 the literature is derived at second hand from the abstracts 

 published by the Chemical Society of London. I should men- 

 tion, however, that Professor Ostwald's recent book, "Out- 

 lines of General Chemistry," which has been translated into 

 English by a younger worker in the same field, Dr. J. Walker, 

 of the Edinburgh University, contains an admirable exposi- 

 tion of the theory by a master-hand. Having myself derived 

 untold pleasure and profit from the perusal of this book, I feel 

 that I shall have done some good if by this address I do no 

 more than induce others to read it too. 



Briefly stated, the new theory amounts to this : that 

 matter in the state of solution is subject to exactly the same 

 laws of volume, pressure, and temperature as regulate matter 

 in the gaseous state, and that the part played by the solvent 

 in the former case is similar to that played by space in 

 the latter. Imagine two vacuous chambers of equal size, 

 capable of being placed in communication with one another or 

 separately with the air. Call one A, and the other B. 

 Now allow a certain quantity of air to enter A. It will, 

 according to its quantity and the size of the chamber A — 

 that is to say, according to its concentration — exercise a 

 definite pressure. If now A be ]3ut in communication with 

 B, part of the air will enter B in virtue of this pressure ; and 

 the redistribution will be such that in a little time A and B will 

 have divided the air equally between them ; and this air, filling 

 twice the space it did before, will have only half the original 

 concentration and therefore only half the original pressure. 

 Now imagine the same two chambers filled, A with a solution 

 of salt in water, and B with pure water. Both chambers are, 

 as a matter of fact, filled with water, just as they were filled in 

 the other case with space ; and throughout A there is also 

 distributed a definite quantity of salt particles, comparable to 

 the air which we then allowed to enter. This salt, like the 

 air, must possess a definite concentration, determined by its 

 amount and the size of A — that is to say, its amount and the 

 volume of the solution ; and in right of this it will exercise a 

 definite pressure, just as the air did. And so, when we open 

 the door from A into B, the salt will gradually become equally 

 distributed throughout both chambers, and by the time that 

 equilibrium is established it will possess but half the concen- 

 tration and half the pressure that it did before. This is the 

 phenomenon so long known as difi'usion, the term being 

 applied to gases and to solutions alike. The other terms 



