176 METALS AND THEIR COMBINATIONS. 



A similar decomposition takes place when alkaline hydrates 

 are added to the 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: 



Hg 2 Cl 2 + Ca2HO = CaCl 2 +' H 2 O -f Hg 2 O. 



Mercuric oxide, Oxide of mercury, HgO = 215.7. There are two 

 mercuric oxides which are officinal ; they do not differ in their 

 chemical composition, but in their molecular structure. 



The yellow oxide of mercury, Hydrargyrum oxidumfiavum, is made 

 by pouring a solution of mercuric chloride into a solution of 

 potassium hydrate, when an orange-yellow, heavy precipitate is 

 produced, which is washed and dried at a temperature not ex- 

 ceeding 40. 



The red oxide of mercury, Hydrargyrum oxidum rubrum, or red 

 precipitate, is made by either heating mercuric nitrate by itself, 

 or after this has been intimately mixed with an amount of 

 metallic mercury equal to the mercury in the nitrate used. In 

 the first case, nitrous fumes and oxygen are given off, mercuric 

 oxide remaining: 



Hg2N0 3 = HgO + 2N0 2 + O. 



In the other case, no oxygen is evolved : 



Hg2N0 3 + Hg = 2HgO + 2NO 2 . 



The red oxide of mercury differs from the yellow oxide in 

 being more compact, and of a crystalline structure ; while yellow 

 oxide is in a more finely divided state, and consequently acts 

 more energetically when used in medicine. Yellow oxide when 

 digested on a water-bath, with a strong solution of oxalic acid, 

 is converted into white mercuric oxalate within 15 minutes, 

 while red oxide is not acted upon by oxalic acid under the same 

 conditions. 



"When mercuric chloride is added to lime-water, a mixture is 

 formed, holding in suspension a yellow oxy chloride of mercury; 

 this mixture is known as yellow-wash. 



Mercurous chloride, Hydrargyrum chloridum mite, Hg 2 Cl 2 = 

 470.2 (Calomel, Mild chloride of mercury, Subchloride or protochloride 



