. of Substances in the Liquid and Solid Condition. 519 



tion which give the heat of combination are contained in the 

 first two columns. The latter, of course, apply to the 

 temperatures at which the heat of dissolution was determined, 

 and this varies in different cases from 19° to 15°. 



We may conclude with a fair degree of confidence that, 

 even in the case of the most stable compound investigated, its 

 constituents are dissociated to a certain extent in the liquid 

 condition, and, therefore, that the heat evolved on mixing the 

 liquids should be less than that evolved on combining the 

 solids, but, it will be seen, that in the first four instances 

 the reverse is the case, the apparent heat of combination 

 of the liquids is the greater of the two quantities, and we 

 must therefore conclude that this apparent heat of combi- 

 nation is not the true heat of combination, and that the 

 hypothesis on which it was taken to be the true heat of com- 

 bination — namely, that the heat of fusion of the compound is 

 equal to the sum of that of its constituents — is incorrect (see 

 p. 510). In order to account for the apparent heat of com- 

 bination of the solids (H'j being less than that of the liquids 

 (H), it is necessary, as will be seen from the equations given 

 on pp. 510, 511, that the heat of fusion of the compound should 

 be less than the sum of those of its constituents, and this is the 

 direction in which we should naturally expect inequality, if any 

 such exist : for when two substances have expended some of 

 their residual affinity in combining with each other, they will 

 have a smaller supply available with which to combine with 

 themselves, when they form aggregates on passing into the 

 solid condition, and their heat of fusion when combined will, 

 therefore, be less than when uncombined. The only one of 

 the seven substances examined in which the heat of com- 

 bination in the liquid form is considerably less than that' in 

 the solid is the hydrate of pinacone : this does not, of course, 

 necessarily imply that it is an exception as to the heat of 

 fusion of the compound being less than the sum of those of 

 its constituents, but simply that the hydrate may be dis- 

 sociated to a very large extent on fusion. 



The other two exceptional instances are exceptional also as 

 regards the negative value for the heat of their formation. 

 In the case of benzene combining w r ith azobenzene this has 

 already been mentioned, and a probable explanation of it 

 has been offered : a similar fact is noticed in the case of 

 the compound of naphthalene with dinitrobenzene where there 

 are no grounds for suggesting a similar explanation, but the 

 values in this case are so small that the heat of combination 

 might well be a positive quantity within the limits of experi- 

 mental error, the values given depending, as they do, on so 

 many different measurements. 



