The hexahydrated double sulphates containing thallium

The author’s prolonged study of the monoclinic isomorphous series of hexahydrated double sulphates and selenates, R2M(SSeO4)2. 6H2O, in which M may be magnesium, zinc, cadmium, copper, nickel, cobalt, iron (ferrous), or manganese, was completed as regards the salts in which R is the alkali metal potassium, rubidium, cæsium, or the base ammonium, NH4, by the memoir communicated in the year 1922. The metal thallium, in its thallous capacity, is also capable, like ammonium, of isomorphously replacing the alkali metals; but it has been found so difficult to grow crystals of these thallium double salts adequately perfect to yield results comparable in accuracy and completeness to those obtained for the salts of the alkalies, that hitherto only four of the fourteen possible thallium salts of the series have been described by the author, although all but one (thallium ferrous selenate) were prepared and examined by him as long ago as 1909, when thallium zinc sulphate and selenate were described, the crystals of the remainder proving inadequately perfect (distorted or more or less opaque). It was much later, in 1925, that two other of the thallium double sulphates, those containing nickel and cobalt as the M-metal, were described, having at length also been obtained in good crystals. During the last two years the author has returned to the work, and with the large experience gained in the course of this long investigation, has at length succeeded in growing sufficiently perfect crystals of the whole 10 remaining salts, and has been able to effect not only a complete goniometrical measurement but a full optical and volume investigation. The results are communicated in the present memoir, as regards the double sulphates, and in the immediately succeeding paper as regards the double selenates, thus finally completing and rounding off the author’s long task by bringing it to a satisfactory conclusion.