516 On the Heat of Combination of Substances. 



by unequal expansion. Two thermometers were destroyed 

 by heating too rapidly. 



Compound of Benzene and Azobenzene. — The values for 

 benzene will be found in the Proc. Roy. Soc. loc. cit. The 

 solvent used in determining the heat of dissolution was 

 benzene. 



On calculating the results, the heat of combination, both in 

 the liquid and solid condition, was found to be negative. It 

 appears very improbable, however, that this should be a 

 correct result in a case where we have direct combination 

 occurring in the absence of a solvent, and without the forma- 

 tion of any secondary products : in every case, except one 

 doubtful one, where the heat of formation of a hydrate has 

 been measured, the value is a positive quantity (see Chem. 

 Soc. Trans. 1887, p. 77). The explanation here may be that 

 the measurement of the heat of fusion of benzene and of the 

 compound is at fault : in both cases the heat-capacity in the 

 solid condition is greater than that in the liquid condition, 

 which is exceptional and which is generally taken to imply 

 that the heat absorbed in fusion is absorbed gradually over an 

 appreciable range of temperature, and not all at the ordinary 

 fusing-point, so that the heat of fusion as measured at the 

 fusing-point itself is too low, and the heat-capacity of the 

 solid as measured in its neighbourhood is too high. 



Compounds of Naphthalene with Metadinitrobenzene and 

 with Dinitrobenzene. — These compounds, and several others of 

 a similar character, were obtained by Hepp (Annalen^ ccxv. 

 p. 379) by mixing solutions in benzene of the two constituents 

 and crystallizing. The presence of any solvent appeared, 

 however, to be superfluous, and preparations of them were 

 made by mixing the substances in the proper proportions 

 when liquid. It seemed desirable, however, to obtain more 

 evidence than that heretofore existing as to the substances 

 formed being really definite compounds. Series of freezing- 

 point determinations were, therefore, made, the results of 

 which are given in Table V. and figs. 2 and 3. These prove 

 the definite existence of the compounds in question : we have 

 a figure made up of three curves (fig. 2), the first one 

 representing the lowering of the freezing-point of dinitro- 

 benzene by naphthalene, the last the lowering of the freezing- 

 point of naphthalene by dinitrobenzene; the intermediate 

 one is evidently an independent curve with two branches and 

 represents, therefore, the crystallization of some third sub- 

 stance ; the maximum of this curve is situated at 57 per cent, 

 of dinitrobenzene, which agrees well with equimolecular pro- 

 portions, these requiring 56*7 per cent. 



The results in the case of dinitrotoluene are similar, except 



