518 Mr. S. U. Pickering on the Heat of Combination 



that the range of strength over which the compound crys- 

 tallizes is considerably greater. Here the maximum of the 

 curve is at about 59*5 per cent., equimolecular proportions 

 requiring 58*7 per cent. 



In the heat of dissolution determinations it was necessary 

 to use a solvent — benzene — which was not one of the con- 

 stituents of the compounds, a slight modification in the 

 ordinary procedure was therefore necessary. Since the 

 heat of formation of the compound AB is given by the 

 difference between the heat of dissolution of AB and of A and 

 B separately, the calorimetric liquid must have the same 

 composition at the end of each of the determinations or the 

 reactions will not be comparable, the heat of dissolution A 

 must be determined in a solvent containing already a corre- 

 sponding proportion of B ; similarly with the heat of dissolution 

 of B. An equivalent proportion of naphthalene was, there- 

 fore, added to the benzene before determining the heat of 

 dissolution of nitrobenzene or nitrotoluene in it, and equivalent 

 proportions of either of the latter added before determining 

 that of the naphthalene. In the table will also be found the 

 values for the heat of dissolution of the various constituents 

 in pure benzene, although these are not of any service for the 

 present calculations. 



The values for the heat-capacity and heat of fusion of 

 naphthalene have already been given in the Proc. Roy. Soc. 

 loc. cit. 



Table II. contains also values for the heat of fusion and 

 heat-capacity of aluminium bromide. These were determined 

 because it was thought that the hydrate with 12H 2 would 

 prove a suitable instance for investigation, since Roozeboom 

 states that it melts at a few degrees above 100° *. It was found, 

 however, that in melting it partially decomposed, forming 

 some less fusible compound, for the liquid was always cloudy, 

 and would not become clear even on boiling. Whatever the 

 substance is w r hich thus separates, it appears to enter into 

 combination again on cooling, for the addition of a little 

 water forms a clear solution, no basic salt or alumina being 

 apparently present. This behaviour was observed both with 

 a preparation made by the author and with one made by 

 Messrs. Kahlbaum. 



Results Obtained. 



The values for the apparent heat of combination in the two 

 conditions are given in Table VI., arranged in the order of 

 magnitude of the heat of combination per molecule of water, 

 or analogous constituent. The values for the heat of dissolu- 



* I have unfortunately mislaid the reference to the paper containing 

 this statement. 



