xxiii 



This remarkable property of gases was viewed by Graham as a result 

 of one of the initial endowments of matter, and in this search he 

 showed his usual desire of approaching nearer than we had ever done to 

 the actual constitution of the primitive molecule and practical atom. 

 These inquiries on the motion of gaseous molecules led Graham to look at 

 the motion of bodies in solution or "liquid diffusion." The first paper was 

 read in 1849. He naturally connected this with his earlier experiments on 

 the phosphates, and the amount of water held by them and phosphoric 

 acid ; and he termed solubilities of substances weak and strong, as well as 

 great or small. He supposed that this varying strength of solubility 

 might arise from a greater or less diffusive power. 



Graham believed liquid diffusion to have an analogy to evaporation ; 

 and as the squares of the times of equal diffusion of gases are in the ratio 

 of their densities, so by analogy it might be inferred that the molecules 

 of the several salts, as they exist in solution, possess densities which are 

 to one another as the squares of the times of equal diffusion. He attri- 

 buted the diffusion of substances in solution, like the transpiration of 

 gases, to a fundamental property of bodies. The pith of the inquiry is 

 thus stated : — " The fact that the relations in diffusion of different sub- 

 stances refer to equal weights of these substances, and not to their atomic 

 weights or equivalents, is one which reaches to the very basis of molecular 

 chemistry. In liquid diffusion we deal no longer with chemical equiva- 

 lents or the Daltonian atoms, but with masses even more simj)ly related to 

 each other in weight. Founding still upon the chemical atoms, we may 

 suppose that they can group together in such numbers as to form new and 

 larger molecules of equal weights for different substances ; or, if not of 

 equal weight, of weights which appear to have a simple relation to each 

 other. It is this new class of molecules which appears to play a part in 

 solution and liquid diffusion, and not the atoms of chemical combination 

 He seems glad to obtain the densities of a new kind of molecules, although 

 knowing no more respecting them. One result is the formation of classes 

 of equidiffusive substances ; these, again, led to a new mode of analysis in 

 1861, and a new division of soluble bodies. It was observed that the 

 power of diffusion of a solution of albumen was very small, 1000 times less 

 than that of common salt ; and this fact led to an examination of numerous 

 substances, when it was found that they divide themselves into two classes 

 without respect to organic nature ; one is colloid, and includes gelatine and 

 gelatinous silica, alumina, albumen, gums, sugar, starch, and extractive 

 matter. The plastic elements of the animal body are found in this class ; 

 and here Graham uses the word (as, indeed, he uses all words) in a very 

 exact sense, that which has the power of forming. 



Continually seeking the origin of chemical action, he ascribes to bodies 

 possessing this colloidal condition a dynamical character. They are slow 

 in changing, but seem in a continual change. They possess energia, and 

 may be the primary source of the force appearing in the phenomena of 



