Cadmium, and Mercury. 379 



The thermal behavior of these substances is confusing. In 

 the first place, cinnabar turns black as the temperature rises. 

 This is not due to any transformation, but merely to a vari- 

 ation in the absorption of light with changing temperature, for 

 after it has been heated to 325° for many hours, cinnabar 

 quickly regains its color on cooling. If, however, the temper- 

 ature is carried to 445°, or perhaps to a lower point, the color 

 remains permanently black. This was first interpreted to mean 

 a transformation into metacinnabar, but a microscopic examina- 

 tion failed to disclose anything but cinnabar, while by grinding 

 in a mortar the black color was found to be due merely to a 

 thin coating. By matching the tint of a groxind sample of 

 heated cinnabar with a ground mixture of the red and black 

 sulphides, the former was found to contain 1 per cent or less 

 of the black sulphide. The same final product was obtained 

 whatever form of mercuric sulphide was heated, provided the 

 temperature was as high as 500° and no further change was 

 found on heating to 550°. This is difficult to explain satisfac- 

 torily. Possibly the coating is due to a condensation of the va- 

 por to the black form. The volume of the tubes was about 05 -cc 

 and the weight of the sulphide taken was half a gram. On the 

 assumption that the tubes were filled with undissociated vapor 



1 16 x ' 09 ms 

 at one atmosphere, they would have contained =5-2 mg 



of sulphide vapor, and if this were all condensed on the sur- 

 face of the sulphide, the mixture would have contained 1 per 

 cent of black sulphide. Apparently little or none condensed 

 on the glass at 500 . Even if the vapor pressure were only half 

 an atmosphere, the results would still be of the right order of 

 magnitude. The results on the /3'-HgS, while conclusive, regard- 

 ing the relation between it and cinnabar, do not settle the rel- 

 ative stability of the two unstable forms. With ammonium 

 sulphide the /3'-HgS appears to be transformed more slowly 

 than the a'-form, though it is very difficult to detect small grains 

 of cinnabar in a matrix of the former. Again, if we were to 

 judge from the behavior of the /S'-HgS when heated in evacu- 

 ated tubes we might conclude that it passed through the a'-form 

 in its transformation to cinnabar. (See Table XL) This would 

 make the /3'-form the least stable which might be expected 

 from the fact that it is not found in nature. However, the 

 a,pparentlxj large percentage of the black sulphide in the /3'-HgS 

 during the early stages of its transformation, may be nothing 

 more than a surface coating on the numerous little grains, which 

 in grinding still remain black because they escape the crushing 

 action of the pestle. At the end of the transformation the 

 /S'-HgS has always changed into comparatively large cinnabar 

 crystals in which, of course, the surface is greatly reduced. 



