MERCUROUS COMPOUNDS. 649 



high metallic lustre. When pure, its surface does not tarnish in 

 air. At 39 or 40 degrees below zero, mercury becomes solid ; it 

 crystallizes in regular octohedrons. According to M. Kupffer, 

 the density of mercury at 39.2 is 13.5886; at 62.6, 13.5569. 

 and at 78.8, 13.535. In the solid state, its density is about 

 14.0. At 662 it boils, forming a colourless vapour, of which 

 the density was observed to be 6976, by Dumas ; the theoretical 

 density is 6978. Mercury emits a sensible vapour between 68 

 and 80, but not under 40. When heated near its boiling 

 point, mercury absorbs oxygen from the air, and forms crystal- 

 line scales of the red oxide. It is not affected by boiling 

 hydrochloric or diluted sulphuric acid, but is readily dissolved by 

 diluted nitric acid. This metal never dissolves in hydrated 

 acids, by substitution for hydrogen. Mercury combines with 

 oxygen in two proportions, forming the black oxide, which is 

 generally considered a suboxide, Hg 2 O, and the red oxide, 

 composed of single equivalents, Hg O, both of which are bases. 

 The equivalent of mercury is then assumed to be 1265.8; but 

 whether it should be this number or a multiple of it by two, we 

 have no certain means of deciding, while in ignorance of any 

 isomorphous relation of mercury with the magnesian metals. 



MERCUROUS COMPOUNDS. 



Suboxide of mercury (black oxide), Mercurous oxide, Hg 2 O, 

 2631.6 or 210.86. This oxide is obtained by the action of a 

 cold solution of potash, used in excess, upon calomel. They 

 should be mixed briskly together in a mortar, in order that the 

 decomposition be as rapid as possible, and the oxide be allowed 

 to dry spontaneously in a dark place. Mr. Donovan finds 

 these precautions necessary, from the disposition which this oxide 

 possesses, to resolve itself into metallic mercury and the higher 

 oxide. The decomposition of mercurous oxide is promoted by 

 elevation of temperature, and by exposure to light. The 

 mercurous oxide is a black powder, of which the density is 

 10.69 (Herapath) ; it unites with acids and forms a class of salts. 

 It is precipitated as the black oxide by lime-water and the pure 

 alkalies, and by alkaline carbonates as a white carbonate, which 

 soon becomes black from decomposition. Its soluble salts are 

 all partially decomposed by pure water, which combines with a 

 portion of their acid, and throws down a subsalt containing an 

 excess of oxide. They are precipitated black by sulphuretted 



