M. H. Sainte-Claire Deville on Affinity and Heat. 375 



ration is effected. Hence it ought to have all the properties of a 

 dissolved substance. It will soon be seen that these properties 

 are only apparent. 



That is what takes place in the dialyzer ; moreover the hydro- 

 chloric acid passes through the filter, and alumina remains on the 

 surface in the state of apparent solution — or in the colloidal state, 

 to use the expression which Mr. Graham has adopted. Yet in 

 practice this reaction does not take place exactly in this manner. 



We may suppose the permeable membrane divided into two 

 horizontal layers, — one the higher layer, where the hydrochlorate 

 of alumina penetrates (this has little thickness), the other the 

 lower layer, where water alone penetrates. It is clear that in an 

 apparatus of this kind the solution of hydrochlorate of alumina 

 will very soon find a layer near the surface where the water, which 

 is renewed with rapidity, will be in considerable proportion as 

 compared with hydrochlorate. In this layer the decomposition 

 of the salt will then take place by indefinite diffusion (conse- 

 quently by the effect of heat), the hydrated alumina will remain 

 in the state of colloidal particles on the upper surface, and the hy- 

 drochloric acid will be carried away by the water. The colloidal 

 layer thus produced will itself become a true filter, and the phe- 

 nomena of decomposition may go on in its interior. 



Yet decomposition by diffusion cannot be complete ; for it varies 

 with the ratio of the quantity of water put in contact with hy- 

 drochlorate of alumina, and with the quantity of hydrochloric acid 

 which this water contains. In other words, the quantity of 

 water separated by diffusion will be the smaller the more this 

 water is charged with hydrochloric acid (this acid arising from a 

 portion of hydrochlorate already decomposed). Everything takes 

 place as if the heat necessary for entirely decomposing hydrochlo- 

 rate of alumina could never be entirely equal to that which exten- 

 sion furnishes to its molecules, within the limits of our experiments. 



Thus the phenomenon of dialysis is never complete ; a little 

 hydrochlorate of alumina always passes through the membrane, 

 and the colloids cannot be separated in an absolute manner by 

 means of diffusion. 



This principle has numerous applications. If we admit gene- 

 rally that all solutions are cooled 'on being diluted, as is shown 

 by a great number of experiments — if, moreover, we assume, as 

 my experiments seem to show, that in every change of state ac- 

 companying solution there is a loss of vis viva, and hence a con- 

 centration of latent heat in the substances dissolved, and in their 

 menstrua, we can understand a great number of phenomena the 

 explanation of which escapes us, and bring them within the class 

 of known phenomena. 



Thus the sap of trees is, so to say, pure water at the moment 



