SILVER MERCURY. 197 



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

 pounds, distinguished as mercuric and mercurous compounds. 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 O, Hg 2 Cl 2 . In order to explain this 

 behavior we have to assume that of the four points of attraction, 

 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. 



/Cl Hg Cl 



Hg< 



X C1 Hg Cl 



Mercuric chloride. Mercurous chloride. 



There are known, however, some data which seem to contradict 

 this view and make it not unlikely that the composition of mercurous 

 chloride is HgCl, and not Hg 2 Cl 2 . 



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

 chloric acid, while chlorine, bromine, and iodine combine with it 

 directly, and w r arm sulphuric and nitric acids dissolve it. 



Mercury is used in the metallic state for many scientific instruments 

 (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 constituents) ; for manufacturing from 

 it all of the various mercury compounds, and those official prepara- 

 tions 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. Mercury exists in 

 a metallic, but highly subdivided state in these preparations, which 

 are made by intimately mixing (triturating) metallic mercury with 

 the different substances used (viz., chalk, pill-mass, fat, lead-plaster). 

 It is most probable that the action of these agents upon the animal 

 system is chiefly due to the conversion of small quantities of mercury 

 into mercurous oxide, which, in contact with the acids of the gastric 

 juice or with perspiration, are converted into soluble compounds 

 capable of being absorbed. 



Mercurous oxide, Hg- 2 O (Black oxide or suboxide of mercury). 

 An almost black, insoluble powder, made by adding an alkaline 

 hydroxide to a solution of mercurous nitrate : 



Hg 2 (NO 3 ) 2 + 2KOH = 2KN0 3 + H 2 O + Hg 2 O. 



A similar decomposition takes place when alkaline hydroxides are 

 added to insoluble mercurous chloride. A mixture of lime-water and 



