360 Allen and Crenshaw — /Sulphides of Zinc, 



easily soluble sulphide of zinc. Of course the sphalerite could 

 not have precipitated wurtzite directly out of the solution since 

 wurtzite should be the more soluble of the two, but when the 

 acid in the solution had become sufficiently neutralized or 

 used up in any way whatever, the hydrogen sulphide present 

 would precipitate the zinc after the less soluble sulphides of 

 copper. 



In this instance, therefore, the formation of wurtzite is well 

 explained by our synthetic experiments and it will be interest- 

 ing to learn whether other occurrences of wurtzite may not be 

 similarly explained. It appears quite possible that wurtzite 

 may often have been taken for sphalerite on account of the 

 general similarity between the two and the lack of careful 

 examination. 



The origin of " schalenblende," which seems to consist of 

 alternate layers of the two forms of zinc sulphide, is difficult to 

 explain, but the suggestion is made that some systematic alter- 

 nation of conditions may have resulted in the periodic neutral- 

 ization of a slightly acid solution. 



II. The Sulphide op Cadaiium. 



So far as present knowledge goes, sulphur forms with cad- 

 mium only* the monosulphide, CdS. Schifff claimed that a 

 pentasulphide was precipitated from solutions of cadmium salts 

 by potassium pentasulphide, but Buchner^: proved that the 

 precipitate thus formed was merely a mixture of the monosul- 

 phide with sulphur. Cadmium sulphide occurs in nature only 

 as the mineral greenockite which crystallizes in the hexagonal 

 system, but at least two other crystalline forms have been 

 reported as laboratory products. § From several points of view 

 it is of interest to know whether these forms actually exist — 

 especially whether there is one of regular symmetry analogous 

 to sphalerite, the common sulphide of zinc, and a systematic 

 investigation of the question was, therefore, undertaken. 



Thermal behavior of Cadmium sulphide. 



A sample of amorphous cadmium sulphide was first heated 

 in a current of hydrogen sulphide in the manner previously 

 described in this paper (p. 345). The sulphide was raised to a 

 temperature of about 1000° and held there for two hours. 

 After cooling, it was submitted to a microscopic examination, 

 when it was found to be entirely crystalline. Individuals were 



*See Follenius, Zs. anal. Ch., xiii, 411, 1874. 

 fLiebig's Ann., cxv, 68; J. B. Ch., 1860, 84. 

 iChern. Ztg., xi, 1087, 1107, 1887. 



SLorenz, Ber,, xxiv. 1501, 1891. Klobukow, J. pr. Chein. (2), xxxix, 412, 

 1887. Beyerinck, N. J., 1897-8, Beilage Bd. xi, 432. 



