Cadmium, and Mercury. 365 



A product obtained under the same conditions was examined 

 by the microscope. It was too tine to identify ; it consisted of 

 small orange-colored doubly refracting crystals. Such proper- 

 ties as could be determined, therefore, agreed with greenockite. 



Explanation of color differences in Cadmium sulphide. 



Cadmium sulphide is a well-known artist's color, and varies 

 from lemon -yellow to orange-red according to the method by 

 which it is prepared. Follenius,* having proved that there 

 was but one sulphide of cadmium, came to the conclusion that 

 these different tints were due to the variable quantity of 

 occluded impurities which he found the sulphide contained. 

 Buchnerf appears to have disproved this. He gives no quantita- 

 tive data, but asserts that he has examined many hundred prepa- 

 rations and finds that sulphide of different tints may contain 

 the same quantity of impurities while variable amounts of the 

 latter occur in sulphide of the same tint. Buclmer describes 

 two different "forms" of cadmium sulphide, the methods of 

 preparing which are not very clearly given, one of which is 

 lemon-yellow, the other "almost as red as minium." Most 

 products, he says, are mixtures of these two forms. Solutions 

 containing little free acid give with hydrogen sulphide a yellow 

 precipitate which becomes orange when it is heated in acid or 

 alkaline solutions. Buclmer asserts that he has never found 

 crystals in any preparation of cadmium sulphide. He regards 

 the orange or "/3-form" a polymer of the yellow or "a-form" 

 because it always has a higher density. Later Klobukow 

 obtained from Buchner a large number of products and sought 

 to prove whether their difference in color was due to chemical 

 or physical differences. He confirmed Buchner's statements 

 regarding the density of the two supposed modifications. At 

 17°-17-5°, after drying at 105°-107°, the yellow form gave 

 specific gravities varying from 3*906 to 3*927, while the orange 

 form showed specific gravities which varied from 4-492 to 

 4'513. These and other preparations were also examined 

 microscopically by Prof. Haushofer. He found them generally 

 doubly refracting, but makes no specific statements regarding 

 extinction angles, indices of refraction or other constants, 

 except that the crystals of the "a-form" were not crystallo- 

 graphically well determined. He regarded the " a-form " as 

 identical with greenockite, while the " /3-products " probably 

 contained both regular and monoclinic forms. 



Our experiments confirm Buchner's experimental observa- 

 tions in almost all respects, but a microscopic examination of 

 products of many different hues, made in several ways, prove 

 that the differences in color are primarily dependent on the 



*Loc. cit. fLoc. cit. 



