The Heat of Combination of Substances. 511 



taking place in the two stages, we shall have, first, the con- 

 version of the solids into the liquids (heat of fusion =/), and 

 subsequently the conversion of these liquids into the molecular 

 condition ; the heat measured in this case will be 



H' = C — V a — V b + V a b —fa —fb +fab • 



H will be equal to H 7 only \ff a +f b =f ab ; i. e. if the heat of 

 fusion of the compound is equal to the sum of those of its 

 constituents. 



The fact that with hydrates and analogous compounds the 

 chemical combination is of a comparatively feeble character 

 rendered it not improbable that this might be the case ; but 

 there is no reason why it should necessarily be so, and the 

 question can only be settled by direct experiment. 



The present determinations show that it is not so. The 

 above argument, as has been said, applies to a case where the 

 amount of combination is supposed to be as great in the 

 liquid as in the solid condition, — that is, where the heat of 

 fusion of the compound,/^, represents nothing but the mere 

 change from the solid to the liquid condition (true heat of 

 fusion). If, however, the compound undergoes partial dis- 

 sociation on fusion, this dissociation will (generally) involve 

 absorption of heat, and its apparent heat of fusion will be 

 greater than its true heat of fusion. The present determi- 

 nations, however, show that the observed heat of fusion of the 

 compound is generally smaller than the sum of those of its 

 constituents, and, a fortiori, the true heat of fusion of the 

 compound must be smaller still ; therefore, the assumption 

 that f a -\-f b =f ab is untenable, and no conclusions can be 

 drawn as to the amount of dissociation occurring on melting 

 the compound from the measurement of H and H7. 



The heat of combination of two substances in the liquid 

 condition was determined by dissolving each of them, and 

 also the compound, separately in a solvent. D being the heat 

 of dissolution, the heat of combination is 



D a + D 6 -D a6 . 



When, as was generally the case, one of the constituents 

 (say a) was identical with the solvent, D a = 0. 



With solids similar determinations were made ; and where 

 the solvent is identical with one of the reagents, D a is the 

 heat of fusion of the substance a. 



As it was not possible to make the determinations in both 

 conditions directly with each substance, the heat of dissolu- 

 tion in the one condition had to be calculated from that 

 observed in the other by means of the heat of fusion. The 



2 M2 



