106 DIVERS AND SHIMIDZU 



sulphur-dioxide gas, at first slowly, as else some black hypomercurosic 

 sulphite may be formed. Or, to the paste of mercuric oxide strong 

 sulphurous-acid solution containing a little sulphuric acid is added 

 until in excess. The presence of the sulphuric acid prevents any 

 formation of hypomercurosic sulphite. Mercurosic sulphite, thus pre- 

 pared, is a splendent-white, distinctly crystalline, voluminous precipi- 

 tate, insoluble in water and in dilute sulphuric or nitric acid, and 

 when dry comparatively stable, except in being efflorescent. It can 

 also be prepared from hypomercurosic sulphite and mercuric nitrate, 

 but it is then of a pale buff colour, and amorphous : — 



IJg. {S0,),+ Hg {N0,%= Hg,{SO,), + {Hg NO,),. 

 In thus existing in two forms, white and buff coloured, this sulphite 

 resembles mercurous chloride. Like the white, the buff-coloured 

 variety is insoluble in dilute sulphuric or nitric acid. By hydrochloric 

 acid mercurosic sulphite is decomposed giving both chlorides of mer- 

 cury. In other respects, too, it behaves as a compound half-mercurous, 

 half-mercuric ; thus potassium hydroxide yields with it mercuric potas- 

 sium sulphite and mercurous oxide : — 



Ifg^ {SO,), + 2 KOH - Hg {SO, K), + Hg, + H,0. 

 Heated with water, it suddenly decomposes into metallic mercury and 

 sulphuric acid ; while when little or no water is present, mercury, sul- 

 phur dioxide, and mercurous sulphate are the products. 



Uiipomcrcurosic sulphite, Hgii^SOs).^, H^O, is best prepared by treat- 

 ing precipitated mercurous sulphate with sulphurous acid, in presence 

 of a little 'sulphuric acid, this acid having the property of preventing 

 decomposition of the hypomercurosic sulphite by the sulphurous acid, 

 just as it prevents that of mercurosic sulphite. Hypomercurosic sul- 

 phite can also be prepared by the addition of sodium sulphite, not in 

 excess, to mercurous nitrate, and in other wiiys. It is a flocculent, 

 voluminous, greyish-black precipitate, insoluble in dilute nitric or sul- 



