388 All a and Crenshaw — Sulphides of Zinc, 



thr induced radial — or parallel — fibrous structure to attain its 

 apparent maximum double refraction. 



The pulverulent precipitate is bright yellow when first 

 formed, and may remain yellow when dry. Conditions which 

 cause aggregation or great increase in size of the particles pro- 

 duce orange-colored powders. The physical characters of 

 powders of various colors are considered in a succeeding sec- 

 tion. Amorphous cadmium sulphide has not been observed in 

 a hard, brittle, doubly refracting form like zinc sulphide. 

 When it exhibits double refraction its refractive index 

 approaches the refractive index of greenockite (crystalline 

 CdS). 



The tendency to crystallize is much greater in some prepara- 

 tions of amorphous cadmium sulphide than in others. This 

 has been observed particularly when these preparations have 

 been embedded in the mixtures of sulphur and selenium pre- 

 paratory to obtaining refractive indices. Large clear globules 

 and lumps from dried flocculent precipitates have not crys- 

 tallized under this condition, but lumpy aggregates of minute 

 globules such as have been formed by heating the latter in 

 strong solutions of sodium sulphide crystallize readily. How- 

 ever, the crystals are oriented at random, and are so minute 

 that only a very strong light reveals their double refraction. 

 On account of the pores in these aggregates their refractive 

 indices could not have been determined were it not for the 

 fact that compressing and moving them about in the viscous 

 mixture causes their surfaces to consolidate into transparent 

 films.* 



Crystalline. — By whatever method produced, the crystals of 

 cadmium sulphide as seen in the microscope were prismatic 

 with parallel extinction, elongation c, pure yellow color, and 

 very faint or imperceptible pleochroism. Crystals from the 

 preparations with molten alkali polysulphide were identified as 

 greenockite by goniometric measurements. Three prismatic, 

 hemimorphic crystals about l : " m long furnished the following 

 data: the prism angles varied between 59 and 61°; 9 angles 

 from the prism to a pyramid varied between 26° 10' and 

 28° 15' ; five of these angles giving the sharpest signals were 

 included between 27° 45' and 28° 15'' ; the base terminating 

 the end bearing this pyramid was dull ; a steeper pyramid with 

 dull faces terminated the other end. The pyramid measured 

 corresponds to 2021 of greenockite. 



The observed refractive indices of these prisms for lithium 

 light are e = 2 - '±4:7, &) = 2'4:25. The crystals from cadmium 

 sulphate, slowly precipitated by hydrogen sulphide, gave a 



*It was at first thought that these films might have taken up appreciable 

 amounts of the constituents of the mixture, but this is not the case. 



