104 DIVERS AND SHIMIDZU 



the same point they diiFer from most sulphites, which, as every chemist 

 knows, are exceedingly easily decomposed by acids. They are not 

 decomposed by water, except Avhen hot, and then they are resolved 

 into metal, whether mercury or silver, and SO3, which with the water 

 forms sulphuric acid, .and this again, where very little water is present, 

 may react with any unchanged sulphite, and give mercurous or silver 

 sulphate and sulphurous acid. This property of decomposing into 

 metal and a non-metallic element, or group of elements, at once recalls 

 the decomposition of mercury or silver cyanide, or gold chloride, by 

 heat. 



One more point of relationship with haloid salts is the particular 

 readiness w4th which mercury and silver sulphites form double sul- 

 phites with very oxidisable metals, such as potassium and sodium, and 

 the comparative stability of these salts. Thus, while a mercury or 

 silver sulphite is decomposed by hot water, and sodium sulphite even 

 by cold water, (if we may judge from the strongly alkaline character of 

 its solution), yet the mercuric (or silver) sodium sulphite is neutral to 

 litmus, and may be boiled with water, or with alkali, without de- 

 composition. Having now discussed some of the points in chemical 

 doctrine, which these bodies serve to throw light upon, we proceed to 

 notice the several mercury sulphites, which in other respects are not 

 without interest. 



Mercuric oxijsulpliite, (OHg., 503)2, H^O, first prepared by Pean 

 de St. Gilles, is best prepared by adding a solution of sodium sulphite 

 to an excess of somewhat concentrated solution of mercuric nitrate, as 

 free as possible from nitric acid. Some mercuric nitrate remaining 

 undecomposed, the oxysulphite almost at once makes its appearance as 

 a flocculent precipitate, rapidly becoming granular and dense. It is of 

 a pale yellow colour, and very unstable. When dry, it is explosive; 

 sudden and complete, though very gentle, detonation being caused by 



