GO 
DR. A. E. H. TUTTON ON THE 
stated as follows:—(l) These ammonium salts are truly isomorphous with the 
similarly constituted potassium, rubidium, and caesium salts of the generic formula 
R 2 M (S0 4 ) 2 .6H 2 0, but are not eutropic with them ; the potassium, rubidium, and 
caesium salts alone form the exclusive eutropic series, in which the crystallographic 
(morphological and physical) properties obey the law of progression with the atomic 
weight of the alkali metal, which has been established in the previous communica¬ 
tions concerning those salts. This law is particularly well illustrated by the fact, to 
which no exceptions have been observed, that the average change of angle between 
the crystal faces, and also the maximum change of interfacial angle (which usually 
occurs for the all-important monoclinic axial angle between the primary faces, 
and exceeds between a K and a Cs salt two whole degrees), are directly propor¬ 
tional to the change in atomic weight, when any one alkali metal is replaced by 
another. 
(2) The dimensions of the space-lattice of any ammonium salt of the series are 
nearly identical with those of the intermediate alkali metal salt, that containing 
rubidium as the R-metal; so that the two atoms of rubidium are replaced by the 
ten atoms of the two NH 4 radicle-groups without appreciably altering the crystallo¬ 
graphic structural dimensions. 
To these may be added the following third general conclusion, which has been 
substantiated as regards thallium in a former communication* (3) The salts of 
this series in which R is thallium resemble the ammonium salts closely, in truly 
belonging to the isomorphous series, but not to the more exclusive eutropic series 
formed by the salts of potassium, rubidium, and caesium. Like the ammonium salts, 
the thallium salts also closely resemble the rubidium salts, but the thallium salts are 
distinguished optically, by their transcendent refractive power, both their refractive 
indices and their molecular refraction being far higher than for any other salts of 
the whole isomorphous series. 
An interesting memoir has recently appeared by F. M. Jaeger f on the crystallised 
ethylsulphates of the rare earths, Rf" (S0 4 . G,H 5 ) (! + 18H 2 0 in which R is Yt, Ce, 
La, Nd, Pr, Sm, Eu, Dys, Gd, Er, Thu, and NYb. Jaeger finds that the crystalline 
forms of these salts are so extremely close to each other that the differences fall 
within the limits of experimental error. He concludes, therefore, that they are 
rigorously identical, oscillating about a mean form in a manner entirely accidental. 
The rare earths, forming a group to themselves, and differing so little (especially 
in their sub-groups) in the values of their atomic weights, present an altogether 
different order of problem to the comparison of K, Rb, and Cs compounds. The 
series of rare earths belong to the central groups of the periodic system, in which 
the electro-positive property is at its minimum, and the chemical characters are 
so remarkably alike that extraordinary processes and refinement of method 
* ‘Roy. Soc. Proc.’ A, 1909, 83, 211. 
t ‘Recueil des Trav. Chim. des Pays-Bas,’ 1914, 33, 343. 
