PHILOSOPHICAL TRANSACTIONS. 
I. The Bakerian Lecture. — On the Diffusion of Liquids. 
By Thomas Graham, F.R.S., F.C.S. 
Received November 16, — Read December 20, 1849. 
Any saline or other soluble substance, once liquefied and in a state of solution, is 
evidently spread or diffused uniformly through the mass of the solvent by a sponta- 
neous process. 
It has often been asked whether this process is of the nature of the diffusion of 
gases, but no satisfactory answer to the question appears to be obtained, owing, I 
believe, to the subject having been studied chiefly in the operations of endosmose, 
where the action of diffusion is complicated and obscured by the imbibing power of 
the membrane, which is peculiar for each soluble substance, but no way connected 
with the diffusibility of the substance in water. Hence also it was not the diffusion 
of the salt, but rather the diffusion of the solution, which was generally regarded. 
A diffusibility like that of gases, if it exists in liquids, should afford means for the 
separation and decomposition even of unequally diffusible substances, and being of a 
purely physical character, the necessary consequence and index of density, should 
present a scale of densities for substances in the state of solution, analogous to vapour 
densities, which would be new to molecular theory. 
M. Gay-Lussac proceeds upon the assumed analogy of liquid to gaseous diffusion 
in the remarkable explanation which he suggests of the cold produced on diluting 
certain saline solutions, namely, that the molecules of the salt expand into the water 
like a compressed gas admitted into additional space. 
The phenomena of solubility are at the same time considered by that acute philo- 
sopher as radically different from those of chemical affinity, and as the result of an 
attraction which is of a physical or mechanical kind. The characters indeed of these 
two attractions are strongly contrasted. Chemical combination is uniformly attended 
MDCCCL. B 
