MERCURY. 323 



sodium, or common salt. It is precipitated in the form of a pon- 

 derous white powder, which must be washed and dried by a gentle 

 heat. Calomel, when pure, has a pale straw colour ; it is per- 

 fectly insipid, inodorous, and nearly insoluble in water. Its 

 specific gravity is 7*2. When exposed to heat, it sublimes unal- 

 tered ; and when the surface of sublimed calomel is scratched, it 

 always exhibits a yellow streak. It is decomposed by lime-water 

 and the alkalies, which instantly render it black, the protoxide of 

 mercury being one of the results ; it is also decomposed by some 

 of the metals, and by the sulphurets of potass and of antimony. 

 Nitric acid dissolves calomel, converting it into corrosive sublimate. 

 It is composed of one proportional mercury 200, and one proportional 

 chlorine 3f), giving 236, as its equivalent representative number. 

 Calomel is one of the mildest and most useful of the mercurial 

 preparations. " It is not so much employed as a remedy in 

 syphilis, principally from its being liable to induce purging ; . but 

 when this is obviated by the addition of small doses of opium, it is 

 given in the dose of one or two grains, morning and evening, and 

 soon affects the general system. It is the mercurial, however, 

 which is chiefly employed in the treatment of other diseases in 

 which mercury is prescribed. To the treatment of some of them 

 it is peculiarly adapted by its action on the intestinal canal, and 

 the secreting organs connected with it; hence, its employment 

 in febrile affections, in hepatitis, and chronic induration of the 

 liver, in scirrhous of other visceral organs, in dysentery, and as a 

 remedy in worms. The mildness of its operation rendering it 

 safe to administer it in large doses, so as rapidly to bring the 

 system under the action of mercury, renders it equally proper for 

 administration in tetanus, hydrophobia, croup, and other diseases 

 in which it is required. The same mildness adapts it to con- 

 tinued use, and hence, the preference given to it in cutaneous 

 affections, in glandular obstructions, in dropsy, and wherever 

 mercury is employed as an alterative. It not only produces the 

 general effects of a mercurial, but also, when given in sufficient 

 doses, acts with certainty and mildness as a cathartic. It is hence 

 often employed to promote the operation of other cathartics, and 

 it has the peculiar advantage, that it does so without adding to the 



