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VIIT Monoclinic Double Selenates of the Iron Group. 
By A. E. H. Tutton, D.Sc., M.A ., F.R.S. 
Received February 2,—Read February 21, 1918. 
The investigation of the crystals of the four salts of the series R 2 M(Se0 4 ) 2 .6H 2 0, in 
which M is ferrous iron, and If is potassium, rubidium, caesium, and ammonium, has at 
last been successfully and completely accomplished, both morphologically and optically, 
and the results are presented in this communication. The difficulties which have 
delayed it are described in a separate memoir, read at the same meeting of the Royal 
Society and published in the ‘ Proceedings,together with details of the manner in 
which these difficulties have been overcome. 
The main obstacles were (l) the difficulty of preparing ferrous selenate FeSe0 4 , a 
salt very much less stable than ferrous sulphate FeS0 4 ; (2) the ready oxidation of its 
solution in water and of the solutions of the double selenates obtained by the addition 
of the calculated quantity of the alkali selenate, and (3) the especial difficulty under¬ 
lying the formation of potassium ferrous selenate, and the fact that even when formed 
the crystals of this salt decompose and become opaque like porcelain within a very few 
hours. 
There is no record of rubidium or caesium ferrous selenates having ever been 
previously either prepared or investigated, crystallographically or physically. 
Ammonium ferrous selenate was prepared by H. ToPS0E,t and its goniometry and a 
density determination were described by him in his 1870 dissertation; its optics were 
also to a very limited extent described by him in conjunction with C. Christiansen, 
in their well-known memoir of 1874. Potassium ferrous selenate offered such 
insuperable difficulties, however, that Tops0E was only able to obtain four approxi¬ 
mate measurements of exterior angles, and no attempt at a density determination 
was made; for, as he states in his 1870 dissertation, he found that the crystals which 
he obtained decomposed immediately after their removal from the mother liquor. The 
salt was consequently never able to be handed over to Prof. ChristiansenJ for optical 
investigation, and is entirely omitted from their joint 1874 memoir. 
* 1 Roy. Soc. Proc.,’ A, vol. 94, p. 352 (1918). 
t ‘ Krystallogr.-kem. Unders. over de selensure Salte,’ Copenhagen, 1870. 
\ ‘Ann. Chim. Phys.,’ series 5, vol. 1, p. 1 (1874). It is with great regret that the author learns of 
the death of Prof. Christiansen on December 28, 1917. 
vol. ccxviii.—a 568. 3 g 
[Published June 17, 1919. 
