MERCURIC SALTS CHANGE INTO EACH OTHER. 193 



existence than mercurous cyanide. Continental writers, generally, 

 have accepted Barfoed's results and state that mercurous sulphide is 

 unknown. Examples may be seen in the English translations of 

 Fresenius and Menschutkin. Kraut-Jörgensen's Gmclin adopts Bar- 

 foed's account of the matter. 



The argument in disproof of the existence of mercurous sulphide 

 is essentially that the precipitate caused by hydrogen sulphide in mer- 

 curous nitrate has not been shown to differ in any property from a 

 mixture of mercuric sulphide and mercury. In some attempts to 

 make a quantitative separation of the mercury and mercuric sulphide, I 

 gfot some results which seem worth recording. When the black 

 precipitate is digested with cold nitric acid, even only strong enough 

 just to act upon it, it quickly begins to whiten, in consequence of being 

 converted into mercuric-sulphide-nitrate. To try to avoid this result, 

 some of the fresh precipitate, washed and moist, was treated, on a filter 

 of SS hardened paper, five times with small quantities of nitric acid, 

 sp. gr. 1.2, the acid being drawn through as quickly as possible by a 

 filter pump. The last portion of acid went through free from mercury 

 and the precipitate had preserved its black colour ; but the mercury 

 which had been dissolved out by the nitric acid did not reach to a half 

 but only -10 per cent, of the whole. The precipitate was, therefore, 

 washed free from nitric acid and then stirred up with hydrogen sul- 

 phide-water, which extracted nitric acid from it, and thus showed that, 

 though not changed in colour, it had yet come to contain some com- 

 bined mercuric nitrate. Complete separation of the mercury from the 

 mercuric sulphide by nitric acid seems therefore not to be practicable. 

 The nitrous acid formed in the dissolution of the mercury is no doubt 

 the active agent in causing some of the mercurous nitrate to become 

 mercuric nitrate in contact with mercuric sulphide. 



The fresh black precipitate, I found to be quickly acted upon by 



