SOME CONSEQUENCES OF GRAHAM'S WORK 603 



the intensity of the solvent attraction, quite irrespective of 

 quantity dissolved. . . . 



" Besides being said to be small or great, the solubility of 

 a substance has also therefore to be described as weak or 

 strong. 



11 The gradations of intensity observed in the solvent force 

 are particularly referred to, because the inquiry may arise how 

 far these gradations are dependent upon unequal diffusibility ; 

 whether indeed rapidity of diffusion is not a measure of the 

 force in question." 



The most important conclusions at which he ultimately 

 arrived was that liquid diffusion supplies the densities of a new 

 kind of molecule but nothing more respecting them. And on 

 this he remarks : 



"The fact that the relations in diffusion of different sub- 

 stances refer to equal weights of those substances and not to 

 their atomic weight or equivalents is one which reaches to the 

 very basis of molecular chemistry. The relation most frequently 

 possessed is that of equality, the relation of all others most 

 easily observed. In liquid diffusion we appear to deal no longer 

 with chemical equivalents or the Daltonian atoms but with 

 masses even more simply 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 weight 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 solubility and liquid diffusion and not 

 the atoms of chemical combination." 



The question we may now ask is — What is the class of 

 molecules which are concerned in solubility and liquid 

 diffusion ? 



It may appear surprising that, although engaged in following 

 the motions of molecules, Graham apparently neither worked 

 nor thought in terms of molecular magnitude — that he always 

 used solutions of this or that percentage strength. But in his 

 day, the minds of chemists were not possessed by Avogadro's 

 theorem — nor are they even now, perhaps, in any proper way. 



Unfortunately his comparative experiments were made with 

 solutions containing equal weights of the substances diffused ; 

 but as he found that the diffusibility of a salt was practically 

 proportional to the amount present, it follows from his observa- 

 tions that of two salts such as sodium and potassium chloride, 



39 



