Cadmium, and Mercury. 367 



be remembered that the density of the products would be 

 changed a great deal by varying intermixtures of crystalline 

 sulphide which Haushofer says was present in these products, 

 and also by occluded salts. Furthermore, in our experience 

 very fine powders are apt to give densities several per cent 

 lower than the same substance in a coarser form. 



In view of all the facts Buchner's hypothesis of polymeriza- 

 tion appeal's untenable. 



Change in color of Cadmium sulphide with rising temper- 

 ature. — When cadmium sulphide is heated, its color becomes 

 progressively darker, until at the full heat of the Bunsen 

 burner, 806°-900°, it is dark red. On cooling, however, it 

 regains its former color. This behavior is entirely similar in 

 character to that of mercuric sulphide (see p. 379), and like that 

 of the latter is to be attributed to a varying absorption of light 

 with rising temperature rather than to a reversible transforma- 

 tion, for the change is perfectly gradual. To avoid the forma- 

 tion of oxide, the experiments were made in evacuated tubes 

 of Jena combustion glass. At 450° the amorphous sulphide 

 had entirely crystallized in two days' time. 



Only one sulphide of Cadmium.- — We find no convincing 

 evidence either in the literature or in our own experiments 

 that cadmium sulphide crystallizes in more than one form. 

 We have crystallized it by every method we could devise and 

 have obtained greenockite in every case, or where in some 

 instances the crystals were too small to identify, what proper- 

 ties could be determined agreed with greenockite. 



III. The Sulphides op Mercury. 



There are two sulphides of mercury occurring naturally, the 

 well-known cinnabar and the much rarer metacinnabar. In 

 addition to these there is another form, probably hexagonal, 

 not known in nature. All have the same composition, HgS. 



Cinnabar, cr-HgS, is readily formed in the wet way by the 

 action of the soluble alkali sulphides on the amorphous black 

 sulphide of mercury. The most satisfactory preparation 

 method is to heat the latter in a sealed tube at 100° with con- 

 centrated ammonium sulphide. In a short time the product 

 changes to a vermilion color, but it is best to continue the 

 action for 24 hours to insure a thorough transformation. 



The amorphous mercuric sulphide which we used in our 

 work was carefully analyzed. It contained no other metal 

 than mercury except a trace of iron. A small quantity of free 

 mercury and some chlorine, probably in the form of HgCl 2 .2tIgS, 



