MERCURY. 175 



but is slightly volatile at all temperatures: it is almost silver- 

 white, and has a bright metallic lustre. 



Mercury is peculiar in that its molecule contains but one 

 atom, at least when in the state of a gas; in the liquid and 

 solid state it may contain two atoms, like most other elements, 

 but we have as yet no means of proving this fact. 



Mercury is bivalent, and forms, like copper, two series of 

 compounds, distinguished as mercuric and mercurous com- 

 pounds ; in the former, one atom of mercury exerts its bivalence, 

 as in HgO, HgCl 2 , in the mercurous compounds two atoms of 

 mercury exert the same valence, as in Hg 2 0, Hg 2 Cl 2 . We have 

 to explain this fact by assuming that of the four points of attrac- 

 tion, represented by the two atoms of mercury, two are required 

 to hold together or unite these two atoms so as to leave but two 

 for other elements. 



Mercury is not affected by the oxygen of the air, nor by 

 hydrochloric acid, while chlorine, bromine, and iodine combine 

 with it directly, and sulphuric and nitric acids dissolve it. 



Mercury is used in the metallic state for many physical in- 

 struments (thermometer, barometer, etc.); in the silvering of 

 looking-glasses, which is effected by means of an amalgam of 

 tin (amalgams are alloys, in which mercury is one of the con- 

 stituents); for manufacturing from it all the various mercury 

 compounds, and those officinal preparations in which mercury 

 exists in the metallic state. 



These latter preparations are : Mercury with chalk, blue mass or 

 blue pill, mercurial ointment, and mercurial plaster. In these prepa- 

 rations, which are made by intimately mixing (triturating) 

 metallic mercury with the different substances used (viz., chalk, 

 pill-mass, fat, lead-plaster), mercury exists in a metallic, but 

 highly subdivided state. It is most probable that the action of 

 these agents upon the animal system is chiefly due to the con- 

 version of small quantities of mercury into mercurous oxide, 

 which in contact with the acids of the gastric juice or perspira- 

 tion, are converted into soluble compounds capable of absorption. 



Mercurous oxide, Hg 2 (Black oxide or suboxide of mercury}. An 

 almost black, insoluble powder, made by adding an alkaline 

 hydrate to a solution of mercurous nitrate : 



Hg 2 2N0 3 + 2KHO = 2KNO 3 + H 4 O + Hg 2 O. 



