Mr. A. K. Tiittnn. A Cmparat< Vr//wy ,//:/ 



lk;ilino selenate crystals, prepared as described in the author's 

 memoir on the normal alkaline selenates,* adequate to yield after 

 addition to the calculated amount of the M-selenate sufficient of the 

 double salt for all the purposes of the investigation, was dissolved in 

 the minimum of distilled water. An equal molecular proportion of 

 pure zinc sulphate crystals was also weighed out, dissolved in dis- 

 tilled water, and precipitated by a solution of sodium carbonate. The 

 precipitate of basic zinc carbonate was isolated by decantation and 

 prolonged washing with hot water, and subsequently dissolved in a 

 solution of pure selenic acid, whose strength had previously been 

 accurately determined by titration with a solution of pure anhydrous 

 sodium carbonate of known strength. Sufficient of the acid was run 

 in upon the zinc carbonate to provide for one drop of excess, as this 

 prevents the possibility of a basic salt forming after addition of the 

 selenate of the R-metal. The solutions of the two selenates were 

 mixed and allowed to crystallise spontaneously, taking all the pre- 

 cautions to avoid disturbance enumerated in the previous memoirs. 



The spherical projection given on p. 343 of the double sulphate 

 memoir applies equally to the double selenates. 



POTASSIUM ZINC SELENATE, K.,Zn(SeO 4 ) 1 .,6H.jO. 



A determination of the zinc in a quantity of the crystals employed 

 gave the following result: 0-9152 gramme yielded 0*1396 gramme 

 ZnO, corresponding to 12*24 per cent, of zinc. Calculated per cent. 

 Zn = 12-11. 



Goniometnj. 



Ten excellent crystals were employed, derived from several different 

 crops. 



Habit : Short prismatic to tabular. 



Axial angle ft = 75 48'. 



Ratio of axes 



a : b : c = 0-7458 : 1 : 0-5073. 

 Forms observed 

 a = {lOOJooPco; b = {OlOjco^oo; c = {001}0P; p = {110}ooP; 



p' = {120}oo^2; y = {OllJ^oo; <>' = (Ill} + P;r' = {201} + 2Poo. 



The results of the angular measurements are given in the accom- 

 panying table. 



The habits observed resembled those exhibited in the following 

 figures given in the double-sulphate memoir : Fig. 7 without b faces, 

 fig. 8 without the smaller faces, fig. 20, fig. 21, fig. 24, and fig. 25. 

 * ' Journ. Cheui. Soc.,' 1897, p. 846. 



