190 



S. H ADA; HOW MEECUROUS AND 



siuin chloride in promoting oxidation is even greater than the 

 numbers make it appear, for, in the potassium chloride experiment, 

 there was more dissociation, as shown by the greater separation of 

 metallic mercury. 



Mercury bromides and iodides. 



No new experiments upon the bromides and iodides seemed called 

 for, it having been ascertained by others that mercurous bromide dis- 

 sociates in hot solutions of alkali bromides, while every one knows 

 how readily mercurous iodide decomposes in solution of potassium 

 iodide, and is formed by rubbing mercury and mercuric iodide to- 

 gether. 



Mercurous oxide and carbonate. 



That mercurous oxide, or what passes for it, decomposes more or 

 less at the common temperature into mercuric oxide and mercury has 

 long been generally known, but it remained for Barfoed in 1883 to 

 prove beyond question, that the black substance produced by the 

 action of alkalis upon mercurous salts is really something more than a 

 mixture of mercury and mercuric oxide. From almost the first 

 moment of its production it contains metallic mercury and, therefore, 

 mercuric oxide, but that it is, at first, real mercurous oxide, for the 

 most part, Barfoed has established very conclusively by two of its 

 properties. Its specific gravity in the fresh state is 11.13, while that 

 calculated for mercury and freshly precipitated mercuric oxide in the 

 required proportions is 12.14. Secondly, the substance is oxidisable 

 in the air, while mercury, even finely divided, is not so. When ex- 

 posed to the air long enough all its metallic mercury volatilises, 

 mercurous oxide also disappears, and nothing but mercuric oxide 

 remains. In this change, the mercurous oxide continues, no doubt, to 

 decompose into mercury and mercuric oxide, but much of it certainly 



