In a communication made to the Royal Society in April last (‘Phil. Trans.,’ A, vol. 191, 313) the author described an interference dilatometer, by the use of which, owing to the introduction of compensation for the expansion of the platinum-iridium interference apparatus by means of a disc of aluminium laid on the object, the delicate method of Fizeau is rendered equally sensitive in the determination of the expansion of solid substances, notably crystals, which cannot be obtained in blocks of the relatively large size hitherto required. The method is particularly applicable in the cases of those substances, including the crystals of most artificial chemical salts or other preparations, whose ground surfaces will not take a polish equal to that of glass. The author was led to devise it in order to be able to extend his investigations, concerning the relations between the morphological and physical properties of the crystals of isomorphous series of salts on the one hand and their chemical constitution on the other, to the thermal deformation of the salts in question. In previous communications to the Chemical Society the author has described the results of detailed observations of a large number of morphological and physical properties of the crystals of the series of normal alkali sulphates, containing as metal potassium, rubidium, and cæsium respectively (‘Journ. Chem. Soc.,' Trans., 1894, 628); of twenty-two double sulphates of the series R
2
M(SO
4
)
2
, 6H
2
O in which R is represented by the same three alkali metals (‘Journ. Chem. Soc.,' Trans., 1893, 337 and 1896, 344); and of the normal selenates of these metals (‘Journ. Chem. Soc.,' Trans., 1897, 846). The general result of these investigations has been to show that the whole of the investigated morphological and physical properties of the crystals of these salts exhibit progressive variations which follow the order of progression of the atomic weights of the three alkali metals (K = 39, Rb = 85·2, Cs = 132·7), so that the variations may be said to be functions of the atomic weight of the alkali metal, in the broad sense in which the term “function” is usually applied in connection with atomic weight. Of all the isomorphous series referred to, the normal sulphates alone prove to be suitable for an investigation of the thermal deformation. The double sulphates are unsuitable on account of the ease with which most of them lose water of crystallisation when their temperature is raised, and a similar remark applies to the double selenates, whose investigation with respect to their morphological and physical properties is now proceeding. The simple selenates offer great difficulties on account of their excessively hygroscopic nature, which is so marked in the case of cæsium selenate, in accordance with the rapidly progressive advance in the solubility of the three salts which has been shown (
loc. cit
. p. 851) to follow the order of the atomic weights of the metals, as to place it in the category of effective desiccating agents. The normal sulphate of potassium is absolutely free from this disadvantage, being one of the least soluble of the salts usually classed as soluble in water, 100 cub. centims. of this liquid at the ordinary temperature only dissolving 10 grams of the salt (
loc. cit. p
. 851 and sulphate memoir
loc. cit.
p. 632). Rubidium sulphate is so slightly hygroscopic, its solubility being only 44 per cent., as to present no difficulty on this ground. Cæsium sulphate is decidedly hygroscopic, the solubility being so relatively great as 163 grams in 100 cub. centims. water. Although this characteristic is by no means so strong as in the analogous selenate, the solubility of cæsium selenate being no less than 245 grams in 100 cub. centims. water, still it is sufficiently marked to render the use of the salt for the purpose in question impossible in damp weather. The difficulty has, however, been successfully overcome in the case of cæsium sulphate, by taking advantage of the driest days of the recent remarkably dry summer, and of a few dry frosty ones of the early winter, together with the expedient of utilising the inner chamber of the air bath of the dilatometer as a desiccator, by placing a vessel containing oil of vitriol therein until the actual moment of commencing the observations.
A comparative crystallographical study of the double selenates of the series R
2
M(SeO
4
)
2
,6H
2
O.—Salts in which M is Zinc