884 Allen and Crenshaw — Sulphides of Zinc, 



Different layers of the hardened amorphous product may 

 have very different refractive indices, perhaps indicating dif- 

 ferent compositions. The douhle refraction shown by this 

 amorphous material is probably caused by the strains induced 

 in these nonhomogeneous masses during the process of harden- 

 ing. At temperatures above 200° these amorphous crusts and 

 spherules soon become partly or wholly crystalline, that is, the 

 resulting product has the refractive index, r) u , of 2 - 34. 



Sphalerite. — The isometric crystalline form of zinc sulphide, 

 sphalerite, was produced from solutions and molten salts in 

 the following typical habits : at about 800° from molten sodium 

 chloride it occurred in the form of distorted dodecahedrons 

 about 0T mn long ; at 350° from a concentrated solution of 

 sodium sulphide, dodecahedrons and tetrahedrons 0*01 mni in di- 

 ameter appeared ; at 200° for 11 days in a similar solution, tetra- 

 hedrons alone appeared. The refractive index of these crystals 

 for lithium light determined under the microscope was found 

 to be 2 , 310 ± - 005. Out of acid solutions definite crystals were 

 not observed, but side by side were seen, in some preparations, 

 spherules and crusts of amorphous zinc sulphide, and similarly 

 shaped masses having no double refraction and the high 

 refractive index. The latter were in some cases covered with 

 minute facets. It appears that an amorphous precipitate is at 

 first formed and that this subsequently crystallizes, probably 

 after having hardened. 



Wurtzite. — The hexagonal form, wurtzite, separated from, 

 solutions in three extreme habits : (1) Prismatic, hemimorphic 

 crystals strongly striated across the prism faces, and reaching 

 0*8 mm in length, were produced in the double tube in an acid 

 solution after 2 days at about 375°. (2) Crystals, tabular par- 

 allel to the base and modified by a different pyramid on each 

 end, occurred with these prisms. (3) Hardened amorphous 

 globules from acid solutions at high temperatures become 

 transformed into coherent masses of very small doubly refract- 

 ing grains having the mean refractive index of wurtzite. In 

 two cases distinctly radial-fibrous forms, having the character- 

 istics of wurtzite elongated parallel to the prism, were seen. 

 This structure is found in the natural schalenblendes. Exper- 

 ience has shown that these aggregates, immersed in methylene 

 iodide and viewed in ordinary light, may appear entirely iso- 

 tropic, but mounted in a red mixture of sulphur and selenium 

 having about the same refractive index as the aggregates, they 

 are distinctly doubly refracting in artificial light. 



Wurtzite produced by sublimation may appear in slender 

 needles or small stout prisms. Such crystals made from pure 

 zinc sulphide were used for determining the refractive indices 

 under the microscope. The values obtained are as follows: 



