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VII. Comparative CrystallograpMcal Study of the Double Selenatea of the 
Series IIo M(Se0,)„6H,0. —Part II. Salts in ivhich J\1 is Mayiiesiuin. 
By A. E. Tutton, B.Sc., F.B.S. 
* 
IveceiA^ed April 29,—Head 23, 1901. 
[Plate 9.] 
Ix this memoir are commimicated the results of an investigation of the magnesium 
group of double selenates, in which II is represented hy potassium, ruhidium, and 
cgesium. It is analogous and strictly comparable to that concerning the salts of the 
zinc grouj), which was presented to the Iloyal Society in March, 1900 (‘ Poy. Soc. 
Proc.,’ vol. 67, p. 58). The potassium salt of tlie series was partially studied by 
Topsoe and Christiaxsex,^ in the year 1874, with the less perfect means then 
available; the rubidium and csesium salts have not hitherto been investigated. 
The three salts were prepared precisely as in the case of the zinc salts, except that 
the normal magnesium selenate recpiired for the })roduction of the doulde salt, l)y 
addition to the calculated amount of the normal alkaline selenate, was prepared hy 
digestion of the calculated cpiantity of pure diluted selenic acid with excess of pure 
calcined magnesia and subsecpient filtration. 
The same spherical projection is common to the series of double selenates and that 
of the double sulphates previously investigated by the author, and is given on p. 343 
of the memoir describing the latter, t 
Potassium Magxesium Selexate, KoMg(SeO^)2,GHoO. 
A determination of the content of magnesium in a specimen of the crystals 
employed afforded the following result:—1T430 gramme yielded 0'2G23 gramme of 
magnesium pyrophosphate, corresponding to 4'95 per cent, of magnesium. The 
calculated percentage of Mg is 4'84. 
Goniometry. 
Twelve good crystals of small size, selected out of five crops, were used in the 
measurements. 
* ‘ Ann. de Chim. et de Phys.,’ vol. 1, 1874, p. 75. 
t ‘ Journ. Chem. Soc., Trans.,’ 1893. 
(293) 
36.10 1901. 
