Cadmium, and Mercury. 375 



regular system. The specific gravities of several products made 

 in this way at 25° were, 



I II III IV V Average 



7-592 7-586 7-642 -7-58S 7-56S 7-60 



The variation in these determinations is, perhaps, to he 

 accounted for hy the presence of some amorphous sulphide in 

 varying quantity. The microscope was unable to decide this 

 point. 



The best determinations of the specific gravity of the nat- 

 ural mineral are close to 7*7. Gr. E. Moore,* who first described 

 and named the mineral, said that the specific gravity varied 

 from 7-701 to 7*748 -'owing to intermixed cinnabar." Genth 

 and Penfield,f who obtained the number 7"706 on a specimen 

 of metacinnabar from San Joaquin, Orange County, California, 

 say : " Color iron-black, but many pieces show already a change 

 into ordinary cinnabarite both by a good lens and the reddish 

 black powder which some of the particles yield on pulverizing." 

 It is probable, therefore, that these values are somewhat high ; 

 on the other hand, it is possible that the values obtained on the 

 laboratory product are somewhat low, since it was impossible to 

 prove that the latter was free from amorphous material. True, 

 Moore calls the mineral amorphous, but no other investigator 

 confirms him. On heating alone or more rapidly with ammo- 

 nium sulphide or dilute sulphuric acid, the metacinnabar passes 

 over into cinnabar. This is true for both the natural and the 

 synthetic mineral (see p. 377). 



P'-HgS. It has already been stated that the precipitation 

 of solutions of sodium mercuric chloride by sodium thiosul- 

 phate in the proportion HgCl 2 : 4Na 2 S 2 3 yields a beautiful red 

 sulphide differing from cinnabar. If the product is to be free 

 from a'-HgS, it should be filtered before precipitation is com- 

 plete. For this reason it is wise to begin with pretty concen- 

 trated solutions, 10 to 25 per cent of total salts, and interrupt the 

 operation while the product is still bright red. It has been 

 found by actual test that the addition of a fraction of 1 per 

 cent of black HgS to the first precipitate which forms (the 

 two being thoroughly mixed in a dry condition), can read- 

 ily be detected by the eye. A microscopic examination in this 

 case was futile. Judging by the color test, the following con- 

 ditions gave a product about 99-5 per cent pure. Two solutions 

 of 4:00°° each were prepared ; one containing 40 g. NaCI 

 and 34-4 g. IigCl 2 , the other 125-6 g. JNa 2 S 2 6 3 .5ll 2 0. The 

 solutions should be filtered if not perfectly clear, and the 

 sodium mercuric chloride solution is then added gradually, with 



* This Journal, iii, 36, 1872. 

 f Ibid., xliv, 383, 1892. 



