650 MERCURY. 



hydrogen. The salts of the same oxide are decomposed by 

 hydrochloric acid and soluble chlorides, with precipitation of 

 calomel as a white powder, a property by which they are distin- 

 guished from the salts of the red oxide of mercury. 



The salts of this, and also of the red oxide, are reduced to 

 the metallic state by copper and more oxidable metals, and by 

 the proto compounds of tin. The precipitated mercury often 

 presents itself as a grey powder, in which metallic globules are 

 not perceived, and remains in this condition while humid. 

 Mercury in this divided state possesses the medicinal qualities 

 of the milder mercurials, and has often been mistaken for black 

 oxide. To obtain precipitated mercury, equal weights of 

 crystallized protochloride of tin (salt of tin) and corrosive sub- 

 limate may be dissolved, the first in diluted hydrochloric acid 

 and the second in hot water, and the solutions mixed with 

 stirring. The salt of tin assumes the whole chlorine of the 

 corrosive sublimate, becoming bichloride of tin, which remains 

 in solution, while the mercury is liberated, and forms so fine a 

 precipitate, that it requires several hours to subside. It may be 

 washed by affusion of hot water and subsidence, and slightly 

 drained on a filter, but not allowed to dry. There can be no 

 doubt that it is in this divided state, and not as the black oxide, 

 that mercury is obtained by trituration with fat, turpentine, 

 syrup, saliva, &c., in many pharmaceutical preparations. 



Subsulphuret of mercury, Hg 2 S, is obtained by the action of 

 sulphuretted hydrogen on a solution of the mercurous nitrate 

 or upon calomel, as a black precipitate. This sulphuret is 

 decomposed by a gentle heat, and resolved into globules of 

 mercury and the higher sulphuret. 



Subchloride of mercury, Calomel, Hg 2 Cl, 2974.3 or 238.33. 

 A variety of processes are given by pharmacians for the pre- 

 paration of this remarkable substance. It may be obtained in 

 the humid way, by digesting l| parts of mercury, with 1 part of 

 pure nitric acid, of density from 1.2 to 1.25, till the metal 

 ceases to be dissolved, and the liquid has begun to assume a 

 yellow tint. A solution is also prepared of 1 part of chloride of 

 sodium in 32 parts of distilled water, to which a certain quan- 

 tity of hydrochloric acid is added, and this when heated to near 

 the boiling point, is mixed with the mercurial salt. The mer- 

 cury acquires the chlorine of the common salt, and the sub- 

 chloride of mercury formed precipitates as a white powder, while 



