SILVER MERCURY. 337 



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

 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. 



Amalgams. Alloys containing mercury are termed amalgams. Mercury 

 unites with most metals, and with some of them it forms definite compounds. 

 Dental alloys used for amalgamation are composed chiefly of silver and tin with 

 one or more of the following metals : gold, platinum, copper, zinc. Dental 

 amalgams are made by reducing the alloy ingot to fine shavings or filings, 

 which are triturated with the necessary quantity of mercury to form a plastic 

 mass, which becomes hard or sets. 



The properties most essential for a good amalgam are strength, immutability 

 as to volume, freedom from discoloration, and resistance to the action of the 

 oral secretions. Many formulas have been suggested to obtain these results. 



The addition of either gold, platinum, copper or zinc to a silver-tin alloy 

 facilitates setting, while this is retarded by an excess of mercury or by the em- 

 ployment of an alloy that has been long exposed to the air or has been annealed. 

 The change in form i. e., expansion or contraction which an amalgam under- 

 goes in hardening is very objectionable and difficult to completely overcome. 



The discoloration of amalgams is in great measure due to the formation of 

 sul phides, particularly upon those amalgams in which there is not complete chem- 

 ical union of the metallic constituents. A proper proportion of zinc in an alloy 

 prevents discoloration considerably, making, however, the alloy difficult to amal- 

 gamate, while the presence of copper greatly increases the tendency to discolor. 



Mercurous oxide, Hg^O (Black oxide or suboxide of mercury). An 

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

 to a solution of mercurous nitrate : 



2HgN0 3 + 2KOH == 2KNO 3 + H 2 O -f Hg. 2 O. 



A similar decomposition takes place when alkaline hydroxides are 

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

 mercurous chloride (calomel) is known as black-wash ; when the two 

 substances are mixed, calomel is converted into mercurous oxide, while 

 calcium chloride is formed : 



2HgCl + Ca(OH) 2 = CaCl 2 -f H 2 O + Hg 2 O. 



Mercuric oxide, HgO = 214.38. There are two mercuric oxides 

 which are official ; they do not differ in their chemical composition, 

 but in their molecular structure. 



The yellow mercuric oxide, Hydrargyrioxidumflavum, is made by pour- 

 ing a solution of mercuric chloride into a solution of sodium hydroxide, 

 when an orange-yellow, heavy precipitate is produced, which is washed 

 and dried at a temperature not exceeding 30 C. (86 F.) (Plate IV., 3) : 

 HgCl 2 + 2NaOH = HgO + SNaCl + H 2 O. 



The red mercuric oxide, Hydrargyri oxidum rubrum, is made by 



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