260 DIGESTION. 



quantities of water, that is to say, their coefficients of condensation 

 (see vol. ii, p. 4). Schmidt is therefore disposed to assume the 

 existence of a chemical attraction even in the solution, and a 

 " hydratation" in the case of a solution in water. If we conceive 

 the idea of chemical affinity in the more limited sense which has 

 hitherto generally been attached to it in chemistry, and continues 

 for the most part still to prevail, we do not think that the con- 

 densation which is here observed affords any stringent proof of 

 the activity of chemical attraction in simple solution. We may 

 certainly give any wider extension that we please to the idea of 

 chemical affinity, but a narrower and stricter limitation of an idea 

 can never do any injury, and cannot in the present case be 

 without a certain significance. The mere condensation of two 

 bodies when they are mixed, is no argument for their chemical 

 combination; for if we are unwilling to admit that the con- 

 densation of gases on the surfaces and in the pores of solid bodies 

 presents a proof against this view, Pettenkofer^s admirable investi- 

 gations on metallic alloys must convince us that, in addition to 

 condensation, a development of light may occur, together with an 

 altered condition of many physical properties, without the oc- 

 currence of any true chemical combination. An attraction for the 

 water is manifested when a solid body becomes fluid ; an attraction 

 is manifested when a solid body, having thus become fluid, 

 undergoes condensation with the water; forces of attraction are 

 manifested when different substances require different periods of 

 time in order to spread or diffuse themselves through a given 

 volume of water. 



It might, therefore, be conceived, that tlie degree of this attrac- 

 tion, manifested in the solution of a solid body in water, would 

 admit of determination, either by comparing together the quanti- 

 ties of the bodies which are able to absorb a definite quantity of 

 water, that is to say, by determining the degree of solubility ; or by 

 the calculation of the coefficients of condensation, using the quan- 

 tities of the heat that is developed as a controlling check ; or that we 

 might ascertain the readiness with which a substance is disposed to 

 diffuse itself during perfect rest through a larger quantity of water. 

 It might be supposed, that as all these three momenta speak in 

 favour of an attraction between solid and fluid bodies, the degree 

 of this attraction might be calculated from the different quantities 

 obtainable by the three methods of investigation, and that one 

 method must control another. Such, however, is not the case. It 

 must be regarded as one of the theoretical deficiencies of chemistry, 



