that the range of strength over which the compound crystallizes is considerably greater. Here the maximum of the curve is at about 595 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 constituents 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 Å 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 corresponding proportion of B; similarly with the heat of dissolution of B. An equivalent proportion of naphthalene was, therefore, 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 12H2O 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 which 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 dissoluI have unfortunately mislaid the reference to the paper containing this statement.

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 combination is not the true heat of combination, and that the hypothesis on which it was taken to be the true heat of combination-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 combination of the solids (H') 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 combination 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 dissociated 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 with 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 experimental error, the values given depending, as they do, on so many different measurements.