322 Mr. Donovan on the [Nov. 



tion, the concentration may become so great as to form oxysul- 

 phate. 



Thus it appears that the result of the action of sulphuric acid 

 on mercury does not depend on the relative quantities of these 

 bodies, but on the force of affinity produced by the degree of 

 temperature. 



43. When sulphate of mercury is decomposed by caustic 

 alkaline solutions, a black powder is produced, which has been 

 considered a subsulphate ; but it will be shown presently that 

 this is not the case. There is nevertheless a real subsulphate 

 which has hitherto been overlooked, and of which a knowledge 

 is essential to the right understanding of some of the salts. 



If black oxide be triturated with sulphuric acid (1 , 090), its 

 colour changes to gray, but it does not dissolve. If this powder 

 well edulcerated be boiled in distilled water, and the liquor 

 filtered, a solution of nitrate of barytes will occasion in it a 

 copious precipitate. This is the real subsulphate of mercury. 

 When formed in the cold, its colour its gray : when digested in 

 boiling water, it loses still more of its acid, and becomes green- 

 ish-gray ; but cold dilute sulphuric acid instantly restores its 

 original colour by affording what it lost. Concentrated acid 

 affords it still more, and renders it white. Thus the subsulphate 

 varies in its composition. 



44. This salt may also be formed by mixing solution of nitrate 

 of mercury with sulphate of soda ; a white precipitate appears 

 which is subsulphate, near the point of saturation. Boiling 

 water changes this to greenish-gray, as in the former case. 



45. When mercury is dissolved by heat in its own weight of 

 sulphuric acid, a saline mass is produced, which, Avhen treated 

 with boiling water, gives a fine yellow powder, often mistaken 

 for Turbith mineral. It has this difference, however, that when 

 decomposed by potash, a dark-brown powder is produced— a 

 fact inexplicable without a knowledge of the existence of subsul- 

 phate of mercury. This brown powder, which might be 

 mistaken for a distinct oxide, is a mere mixture of the black and 

 red, arising from the subsulphate and suboxysulphate, which 

 constituted the original yellow powder; and accordingly with 

 muriatic acid, it forms calomel and corrosive sublimate. Turbith 

 mineral, which has been long exposed to light, affords a brown 

 precipitate upon the same principle. 



46. Thus it appears that there are but two combinations of 

 mercury with sulphuric acid, the sulphate and oxysulphate, with 

 their subsalts ; and that all the other varieties are but mixtures 

 of these. To ascertain the composition of the oxides existing in 

 these salts, I decomposed a quantity of sulphate of mercury (39) 

 by means of pure potash ; the black powder was well washed, 

 dried, and freed from metallic mercury by trituration. This 

 oxide, when submitted to analysis in the manner already 

 described (13),. afforded four per cent, of oxygen, which differs 



