1900.] P. C. Ray — Mercurous Nitrite and its Derivatives. 477 



begin with describing a method of preparing the salt, which has been 

 found to be more economical and far less troublesome than the usual 

 method. 



A tall beaker is taken containing nitric acid (sp. gr. 1041) diluted in 

 the proportion of 1 : 4 with water ; mercury is now poured in, care being 

 taken not to fully cover the bottom but to leave an annular or horse- 

 shoe-shaped space. The crystals which are formed on the convex sur- 

 face of the mercury are continually pushed aside into the empty space 

 by the evolution of gases during the initial stages of the reaction. On 

 standing overnight, however, a crust of the nitrite is formed on the 

 surface of the metal, which acts as a protective layer, thereby hindering 

 further action. 



All that is now necessary to do is to incline the beaker gently, when 

 the deposit of the salt slips off into the empty space as explained 

 before, leaving a fresh surface of mercury exposed. This may be repeated 

 4 or 5 times in the course of the day. Instead of inclining the beaker, 

 the layer of crystals may be carefully scraped off the surface with a 

 glass rod. The process may be allowed to go on for a week, resulting 

 in the continuous growth and accumulation of the salt ; the reaction 

 may be started simultaneously in about half a dozen beakers arranged 

 in a row, so as to secure a copious supply. 



In the previous papers* it was recommended that each time a layer 

 of crystals is formed, the mother liquor together with the mercury 

 should be decanted off into another beaker. This is a wasteful method, 

 for as soon as the super-incumbent liquid is removed, torrents of red 

 fumes appear on the surface of the mercury. These red fumes are 

 caused by the combination of nitric oxide with the oxygen of the air. 

 It is the nitric oxide that evidently gives rise to the formation of th 

 nitrite, and its loss has to be guarded against. 



(2) 

 Preparation op chemically pure Mercurous Nitrite. 

 The mercurous nitrite prepared as above will answer well enough 

 for ordinary purposes. It generally contains, however, impurities, chiefly 

 in two shapes. First, the crystalline mass encloses minute globules of 

 mercury which cannot be entirely detached. Secondly, as the salt has 

 to be dried on the porous tile, a portion of the heavy mother liquor 

 consisting of mercurous nitrate dries up along with it. When it is 

 desirable to obtain the salt in a state of absolute purity, it is mixed with 

 a sufficiently large quantity of water and heated to boiling point for some 



* For literature on the subject, see Journal, Asiatic Society, 1896, Pt. ii, p. 1, 

 and Transactions, Chem. Soc. for 1897, p. 338. 



