198 METALS AND THEIR COMBINATIONS. 



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



Mercuric oxide, HgO = 215.8. 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, Hydrargyri oxidum flavum, is made 

 by pouring 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 + 2NaCl + H 2 O. 



The red mercuric oxide, Hydrargyri oxidum rubrum, or red pre- 

 cipitate, is made by heating mercuric nitrate, either by itself or after 

 it has been intimately mixed with an amount of metallic mercury 

 equal to the mercury in the nitrate used (Plate IV., 4). In the 

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



remaining : 



Hg(N0 3 ) 2 = HgO -f 2N0 2 + O. 



In the other case, no oxygen is evolved : 



Hg(N0 3 ) s + Hg = 2HgO + 2N0 2 . 



The red oxide of mercury differs from the yellow oxide in being 

 more compact, and of a crystalline structure ; while the yellow oxide 

 is in a more finely divided state, and consequently acts more energeti- 

 cally 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 fifteen 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 yellow mercuric oxide; this mixture 

 is known as yellow-wash. 



Experiment 35. Heat some mercuric nitrate in a porcelain dish, placed in a 

 fume chamber, until red fumes no longer escape. The remaining red powder 

 is mercuric oxide, which, by further heating, may be decomposed into its ele- 

 ments. 



Mercurous chloride, Hydrargyrum chloridum mite, 

 = 47O.4 (Calomel, Mild chloride of mercury, Subchloride or proto- 

 chloride of mercury). Mercurous chloride, like mercurous oxide, 



