374 M. H. Sainte-Claire Deville on Affinity and Heat. 



be conceived that in an almost indefinite state of extension it 

 might even be decomposed by the heat which it has absorbed at 

 each addition of the solvent. In this manner is to be explained 

 the decomposition by diffusion produced by Mr. Graham, and 

 which serves as the basis of his admirable method of dialysis. 



If you put bisulphate of potash in the internal diffusion-ves- 

 sel (two cylindrical concentric vessels, Graham's first apparatus), 

 you know that the illustrious physicist shows you sulphuric acid 

 separating from neutral sulphate of potash and passing into the 

 external vessel. 



There we have a true decomposition which is necessarily ac- 

 companied by the absorption of a certain quantity of heat. 

 Let us inquire to what it is due. If we add sulphuric acid to 

 sulphate of potash, both in dilute solution, the liquid will become 

 heated ; but the quantity of heat thus produced will always be 

 less than the heat of contraction calculated in the manner I have 

 shown. In fact there will be a transformation of the heat which 

 should have become sensible during the contraction, into latent 

 heat ; and when this has become considerable enough, decompo- 

 sition by diffusion will take place. 



In order that the inverse of the original effect may take place, 

 the combination which is destroyed must take up the quantity 

 of heat which became sensible at the time it was effected. It is 

 the latent heat stored up during solution which furnishes it \ but 

 it is inadequate for completely producing this effect, and thus 

 the phenomenon is only partial, as Mr. Graham has well observed. 



The phenomena of decomposition by diffusion take place there- 

 fore in a continuous manner, like the decomposition of gases by 

 heat, and all that I am about to say respecting dissociation and 

 its tension (which might here be replaced by the ratios of 

 weights) is absolutely applicable to them. 



Whether diffusion takes place in two concentric vessels as I 

 have supposed, or in Graham's new membrane apparatus, the 

 process is the same. 



Suppose that a body with little stability, such as hydrochlo- 

 rate of alumina, a body which is reduced to its elements by the 

 least heat, be introduced into the dialyzer, above the membrane 

 or parchment-paper which serves as a filter with very close meshes. 

 Below this filter is water, which I assume to be constantly pure. 



The hydrochlorate of alumina absorbs the more heat the 

 greater the quantity of water in which it is dissolved, so that 

 at a given moment it may be supposed to contain so much heat 

 that if this heat became sensible and were applied to its elements 

 they would at once separate. At this moment hydrochloric acid 

 becomes free, and hydrated alumina separates in particles ex- 

 tremely divided, which occupy all the liquid in which the sepa- 



