ON ELECTROLYSIS IN ITS PHYSICAL AND CHEMICAL BEARINGS. 381 



For elements and non-electrolytes one would expect the difference to be less 

 marked. 



Table of atomic heats in liquid and solid states, to confirm this : — 



(The iodine figure seems to need revising.) 



Again, when a body melts, if an electrolyte, it becomes partly active, and more 

 energy is thus required than if it were non-electrolytic ; so we may expect the- 

 latent heat of fusion to be in general higher for electrolytes than for elements. 



[Table of latent heats of fusion to show this follows, but the difference is not 

 exceedingly well marked, and there are exceptions.] 



Most chemists are inclined to imagine a force between different bodies called 

 that of affinity. 



On this view the heat produced by chemical processes is the transformation of 

 a potential energy into heat energy. Berthollet, who adheres to such an opinion, 

 was thus conducted to the necessary consequence that chemical compounds have 

 not a constant composition — a result generally admitted to be erroneous (see § 23). 

 Besides, there are other circumstances in favour of the contrary opinion, viz., that 

 the heat produced by chemical processes is analogous to latent heat. Thus nobody 

 can deny the complete analogy between dissociation {e.g., of CaCO^) and the 

 vaporisation of a liquid ; after the researches of Deville, Debray, Troost, Isambert^ 

 Ditte, Naumann, and other noted investigators. 



But the above-given theory conducts of necessity to the latter opinion. So it 

 is from this point of view also in agreement with experimental fact. 



§ 22. Comparison between some found and calculated numbers. 



M. Ostwald has drawn up a table of the values of the coefficients of relative 

 affinity for different acids, taking that of HNOj as 1. The definition given by 

 Ostwald of this coefficient coincides perfectly with that of ' avidity ' given by 

 Thomsen, who has likewise given a table. We reproduce below both these tables 

 together. . . . Sulphuric and oxalic acids are liable to form double salts which 

 complicate matters a little (§10), but at extreme dilution not much, so their numbers 

 vary with concentration (see § 12). 



The coefficients quoted are, for want of better, those applicable when no other 

 body is present ; but other bodies, if present, diminish them, just as concentration of 

 the same body does. And feeble hydrates, being most affected by concentration, 

 are probably most affected by foreign bodies. These feeble hydrates, therefore, may 

 be expected to be really a little weaker than calculation shows. 



The determination of avidity was made by permitting an equivalent of 

 each to share an equivalent of base (usually NaOH) between them ; the ratio of 

 the fractions of the base seized hj each acid is their relative avidity. Two nimibers 

 are calculated for H2SO4, one if it does not, the other if it does, form an acid salt. 

 As for phosphoric acid, we have supposed that its acid salt has only one H replaced 

 by a metal — a supposition which ought not to be pressed, seeing the great excess of 

 free acid present in a reaction. The following are the only acids for which there 

 are data enough to calculate from. The base supposed in the calculation is NaHQ 

 for HNOj, HCl, HI, and HA ; for the others, KHO. 



