602 SCIENCE PROGRESS 



2. The Diffusion of Liquids 



I pass to Graham's work on the diffusion of liquids — that is to 

 say of substances dissolved in water. To discuss this properly 

 would involve a complete discussion of the process of dissolution 

 and of the problems afforded by solutions generally. 



The words with which he opens his first account of his work 

 on the diffusion of liquids in 1849 show that little was known 

 when he began to study the subject : 



"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 spontaneous 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. . . ." 



He evidently expected to find some kind of simple molecular 

 relationship, as he remarks : 



" 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." 



But it is clear that he was aware that a variety of considera- 

 tions should be taken into account, especially the relationship 

 between solubility and diffusibility; this is shown in the 

 following passages : 



" In the consideration of solubility, attention is generally 

 engrossed entirely by the quantity of salt dissolved. But it is 

 necessary to apprehend clearly another character of solution, 

 namely the degree of force with which salt is held in solution or 



sympathise with both sides — with chemist and physicist and, I think, with the 

 biologist also— and could give mathematical form to their ideas without losing 

 sense of proportion. A balanced mind such as his seems to be almost 

 impossible in these days— the burden of knowledge is too great. I make 

 these remarks, as I feel that the fast-spreading disease of specialisation is 

 one that we need to watch most carefully and guard against. No form of 

 cruelty probably is worse or more fatal in its effects than that of " over- 

 educating " genius— perhaps if we did more in our schools in the way of 

 providing opportunity for free mental development, less in the way of forcing 

 lesson-learning, the results would be more satisfactory. 



