338 METALS AND THEIR COMBINATIONS. 



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 + 2N0 2 -f O. 

 In the other case, no oxygen is evolved : 



Hg(N0 8 ), + 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 44. Shake a small knifepointful of mercurous chloride (calomel) 

 with 50 c.c. of lime-water. Note that the chloride instantly turns dark, due to 

 formation of mercurous oxide (see reaction in text). The lime-water must be 

 in excess. 



Add about 1 c.c. of reagent solution of mercuric chloride to 50 c.c. of lime- 

 water and stir. Yellow mercuric oxide is formed at once (see reaction in text). 

 If the lime-water is not in excess, the precipitate will not be pure yellow, but 

 whitish, due to formation of an oxychloride, Hg 2 OCl 2 . 



Caustic alkalies produce the same results as above, but any excess of these 

 stronger alkalies is objectionable for the purposes for which these " washes " are 

 used. 



Experiment 45. Heat some mercuric nitrate in a porcelain dish, placed in a 

 ftime 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, HgCl = 

 233.68 (Calomel , Mild chloride of mercury, Subchloride or proto- 

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

 may be made by different processes, but the article used medicinally 

 is the one obtained (except it be otherwise stated) by sublimation 

 and the rapid condensation of the vapor in the form of powder. 



It is made either by subliming a mixture of mercuric chloride 

 and mercury: 



HgCl, + Hg = 2HgCl. 



