68 Mr. A. I-!. Tutt'in. A Comparative Orystattograph 



Axial angle : /3 = 74 44'. 

 Katio of axes : n : I : < = 0'7431 : 1 : 0-5019. 

 Forms observed: a = {100}ooPco; b = {010}cooo; e = {001 

 p = {110}ooP; <i = {011}pco 

 = {201} + 2Poo; ' = {121} 



The results of the measurements are tabulated in the accompanying 

 table. 



The types observed resembled those of rubidium zinc sulphate, 

 especially that shown in fig. 7 (p. 359) of the double-sulphate memoir, 

 but without the l> faces as a rule. Only a trace of the orthopinacoid a 

 \VH; discovered ; the clinopinacoid b was invariably small when present, 

 and the hemi-pyramid n' was only present on one of the crystals 

 measured. The d faces were fairly well developed, and afforded good 

 images of the signal. Frequently the only faces present were c, p, /, 

 and q. The c faces varied in relative importance from the breadth 

 usually exhibited in the potassium salts to the narrow strip charac- 

 teristic of the caesium salt. 



There is an excellent cleavage parallel to the faces of the orthodome 

 r'{201}. 



Density. The following four determinations were made with 

 independent quantities of the finely-powdered crystals : 



Weight of Sp. gr. 



salt employed. at Mr/4 9 . 



7-8234 2-8596 



6-0506 2-8611 



7-8824 2-8601 



6-7490 2-8608 



Mean ...... 2*8604 



Molecular Volume. = = 219-90. 



Ratios. On combining the axial angle and the ratio of the 

 axes already given, with the molecular volume, the following ratios 

 are obtained : 



X : ^ : <> =6-3062 : 8-4863 : 4'2593. 



Optics. 



/ifulion of Axes of Optical A7////.W'/. The optic axes (optic bi- 

 normals) lie in the plane of symmetry. The sign of the double refrac- 

 tion is positive. 



The following extinction angles were exhibited by two section-plates 



