MERCUROUS COMPOUNDS. 651 



the nitric acid and oxygen are resigned by the mercury to the 

 sodium, which becomes nitrate of soda : Na Cl and Hg 2 O, NO 5 

 = Hg 2 Cl and NaO, NO 5 . The excess of acid in this process 

 is intended to prevent the precipitation of any subnitrate of mer- 

 cury, which the dilution of the nitrate of mercury, on mixing 

 the solutions, might occasion the formation of. Calomel is also 

 obtained by rubbing together, in a mortar, 4 parts of chloride 

 of mercury (corrosive sublimate) with 3 parts of running mer- 

 cury. The mixture is afterwards introduced into a glass bal- 

 loon, and sublimed by a heat gradually increased. Here the 

 chloride of mercury combines with mercury, and the subchlo- 

 ride is produced. The same result is obtained by mixing the 

 sulphate of red oxide of mercury, with as much mercury as it 

 already contains, and about one third of its weight of chloride 

 of sodium, and subliming the mixture. The vapour of the sub- 

 chloride of mercury, in these sublimations, is advantageously 

 condensed by conducting it into a vessel containing hot water ; 

 the vapour of the water then condenses the salt in an extremely 

 fine and beautifully white powder. The product of this opera- 

 tion is recommended by its purity, as well as its minute divi- 

 sion, for chloride of mercury, by which the subchloride is ac- 

 companied, is dissolved by the water. It appears that whenever 

 the subehloride is sublimed, a small portion of it is resolved 

 into mercury and the chloride. To prepare the calomel for 

 medical use, as it is usually condensed in a solid cake, it must, 

 therefore, be reduced to a fine powder, and also be washed with 

 hot water to remove the soluble chloride. 



Subchloride of mercury is obtained by sublimation in four- 

 sided prisms, terminated by summits of four faces. When the 

 solid cake is finely pounded, the salt acquires a yellow tinge. 

 The density of this salt in the solid condition is 6.5 ; in the 

 state of vapour 8200, one volume of which contains one volume 

 of the vapour of mercury and half a volume of chlorine. This 

 salt is so highly insoluble in water, that when the mercurous 

 nitrate is added to hydrochloric acid diluted with 250,000 times 

 its w r eight of water, a sensible precipitate of subchloride of mer- 

 cury appears. When boiled for a long time in hydrochloric 

 acid, this salt is resolved into chloride of mercury which dis- 

 solves, and mercury which is reduced. 



Action of ammonia on subchloride of mercury. The dry sub- 

 chloride was found by Rose to absorb half an equivalent of am- 



