VIII. Monoclinic double selenates of the iron group

The investigation of the crystals of the four salts of the series R2 M(SeO4 )2 . 6H2 O, in which M is ferrous iron, and R is potassium, rubidium, cæsium, 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 FeSeO4 , a salt very much less stable than ferrous sulphate FeSO4 ; (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 underlying 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.