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VI. Monoclinic Double Selenates of the Nickel Group. 
By A. E. H. Tutton, D.Sc., M.A., F.R.S. 
Received and Read November 23, 1916. 
This communication deals with the four double selenates of the series 
R 2 M(Se0 4 ) 2 .6H 2 0, in which M is nickel and R is potassium, rubidium, caesium, 
and ammonium. Potassium nickel selenate and ammonium nickel selenate only have 
been previously investigated, and to a very limited extent, by vom Rath and Tops0E 
and Christiansen ; rubidium nickel selenate and caesium nickel selenate do not 
appear to have formed the subject of either goniometrical or physical investigation. 
Great care has been taken with the preparation of the four double salts in a state of 
purity. The solutions in pure water from which the crystals were deposited contained 
in each case the two simple constituent salts in exactly equal molecular proportions. 
Good crystallised specimens of potassium selenate and ammonium selenate were 
already in the author’s possession, and the selenates of caesium and rubidium were 
prepared by dissolving the pure carbonates of the two metals in the exact quantity 
of pure selenic acid, all trace of carbonic acid being removed by boiling. The nickel 
selenate was prepared in the four cases separately, by dissolving in pure water the 
quantity of nickel sulphate corresponding molecularly to the amount of the alkali 
selenate to be used in the preparation of the double salt, and adding a very slight 
excess of caustic potash solution (freshly prepared from the weighed quantity of 
solid potash) in order to precipitate all the nickel as nickelous hydrate. After 
warming, so as to complete the precipitation, the liquid was allowed to stand 
until the precipitate had settled, when it was washed with a large volume of water, 
six times by decantation and six further times on the filter. The nickelous hydrate 
was then dissolved in the calculated amount of selenic acid, the filtered solution of 
nickel selenate was added to the alkali selenate solution, and the whole allowed 
to crystallise in successive crops by spontaneous evaporation, with all the precau¬ 
tions indicated on p. 9 of the author’s ‘ Crystallography and Practical Crystal 
Measurement.’ 
Potassium Nickel Selenate K 2 Ni(Se0 4 ) 2 .6H 2 0. 
Morphology. 
Some crystals of this salt were measured in the year 1862 by vom Rath. # They 
were of a very simple character, comprising only four forms, and it is somewhat 
‘ Pogg. Ann. der Phys.’ (1862), vol. 115, p. 484, and vol. 116, p. 363. 
VOL. CCXV1I.-A 554. 2 P [Published June 26, 1917. 
