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X. Hie Thermal Deformation of the Crystalliaed Normal Sulphates of Potassium, 
Ruhiclium, and Ccesium. 
By A. E. Tutton, B,Sc, 
Communicated hy Captain Abney, C.B., F.R.S. 
Received January 31,—Read February 16, 1899. 
In a communication made to the Royal Society in April last (‘Phil. Trans.,’ A, 
vol. 191, 313) the author described an interference dilatometer, hy 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 Idocks 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 investi¬ 
gations, 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 csesium respectively (‘Journ. Chem. Soc.,’ Trans., 1894, 
628); of twenty-two double sulphates of the series R 2 M(S 04 ) 2 , 6 H. 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 })]iysical properties of the 
crystals of these salts exhibit progressive variations which follow the order of pro¬ 
gression 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 inferred to, tlie normal sulphates alone prove to be 
17.6.99 
