112 Mr. Spencer U. Pickering on the 



shown by these properties, and of the existence of the hy- 

 drates thus indicated, namely the isolation of a hitherto 

 unknown hydrate (H 2 S0 4 , 4H 2 0), which, on the strength of 

 its being indicated by changes of curvature, had been stated 

 to exist in solution. It may have been thought that this was 

 but a lucky coincidence, although, as I showed (Chem. Soc. 

 Trans. 1890, p. 340) that the chances against such a coinci- 

 dence were considerable : the multiplication of instances of 

 a similar character must, however, render any such expla- 

 nation untenable, and I have already published accounts of 

 three other instances of a still more striking character. KolVs 

 values for the densities of nitric-acid solutions, when plotted 

 out, showed two breaks of a very marked character indicating 

 the existence of two unknown hydrates with H 2 and 3H 2 

 respectively ; and on examining the solutions by cooling them 

 to low temperatures, these two hydrates, and these two 

 only, were obtained in a solid crystalline condition (Chem. 

 Soc. Trans. 1893, p. 436). Similarly Kolb's and also my 

 own values for the densities of hydrogen-chloride solutions 

 showed a break of a most unmistakable character at the 

 composition of a trihydrate ; and this hydrate, hitherto un- 

 known, was isolated from the solution by cooling it. In this 

 case the break in the densities was particularly w T ell marked, 

 since these formed a very nearly straight line from to 40 per 

 cent., whereas from 40 to 44 per cent, the deviation from 

 straightness was very considerable (see fig. 1, A 0)*. It was, 

 therefore, thought probable that breaks of a similar character 

 might be found in the case of hydrogen-bromide solutions. 

 The results, however, disappointed this expectation : the 

 breaks which were observed were of a very feeble character, 

 but, though feeble, they have proved to be satisfactory other- 

 wise, for 1 have succeeded in isolating the two hydrates which 

 they indicate. 



Topsoe's values for the densities of this acid (Ber. 1870, 

 p. 404) when plotted out indicate a change of curvature at 

 about 28'5 per cent. ; but this corresponds to a hydrate con- 

 taining as much as 11H 2 0, the isolation of which in a crystal- 

 line condition would be improbable. His values extend up 

 to solutions of 48 per cent, strength only (or HBr + 5H 2 0), 

 and, therefore, do not cover the region where isolable hy- 

 drates w r ould most probably exist. I consequently resolved to 

 make determinations with stronger solutions. These results, 

 which are given in Table I., extend from 65*2 per cent, down- 

 wards, the determinations in the region already covered by 

 Kolb's results being comparatively sparse. 



* In the case of nitric acid the breaks were equally well marked, owing 

 to the rectilineal character o^ a large portion of the figure. 





