Note 56 bis — Chapter XX · SULPHUR, SELENIUM, AND TELLURIUM

These hydrates are: (a) H2SO4 = SO3H2O (melts at + 10°·4); (b) H2SO4H2O = SO32H2O (crystallo-hydrate, melts at +8°·5); (c) H2SO42H2O (is apparently not crystallisable); (d) one of the hydrates between H2SO46H2O and H2SO43H2O, most probably H2SO44H2O = SO35H2O, for it crystallises at -24°·5 (Note 50 bis); and (e) a certain hydrate with a large proportion of water, about H2SO4150H2O. The existence of the last is inferred from the fact that the factor ds/dp first falls, starting from water, and then rises, and this change takes place when p is less than 5 p.c. Certainly a change in the variation of ds/dp or ds/dt does take place in the neighbourhood of these five hydrates (Pickering, 1890, recognised a far greater number of hydrates). I think it well to add that if the composition of the solutions be expressed by the percentage amount of molecules—r1SO3 + (100 - r1)H2O we find that for H2SO4, r1 = 50, for H2SO42H2O r1 = 25 = 50/2, for H2SO4H2O, r1 = 33·333 = 50·⅔, while for H2SO44H2O, r1 = 16·666 = 50·⅓—i.e. that the chief hydrates are distributed symmetrically between H2O and H2SO4. Besides which I may mention that my researches (1887) upon the abrupt changes in the factor for solutions of sulphuric acid, and upon the correspondence of the breaks of ds/dp with definite hydrates, received an indirect confirmation not only in the solutions of HNO3, HCl, C2H6O, C3H8O, &c., which I investigated (in my work cited in Chapter I., Note 19), but also in the careful observations made by Professor Cheltzoff on the solutions of FeCl3 and ZnCl2 (Chapter XVI., Note 4) which showed the existence in these solutions of an almost similar change in ds/dp as is found in sulphuric acid. The detailed researches (1893) made by Tourbaba on the solutions of many organic substances are of a similar nature. Besides which, H. Crompton (1888), in his researches on the electrical conductivity of solutions of sulphuric acid, and Tammann, in his observations on their vapour tension, found a correlation with the hydrates indicated as above by the investigation of their specific gravities. The influence of mixtures of a definite composition upon the chemical relations of solutions is even exhibited in such a complex process as electrolysis. V. Kouriloff (1891) showed that mixtures containing about 3 p.c., 47 p.c. and 73 p.c. of sulphuric acid—i.e. whose composition approaches that of the hydrates H2SO4150H2O, H2SO46H2O and H2SO42H2O—exhibit certain peculiarities in respect to the amount of peroxide of hydrogen formed during electrolysis. Thus a 3 p.c. solution gives a maximum amount of peroxide of hydrogen at the negative pole, as compared with that given by other neighbouring concentrations. Starting from 3 p.c., the formation of peroxide of hydrogen ceases until a concentration of 47 p.c. is reached.