SOLUBLE SULPHOSALT OF MERCUKY. 423 



this ooinpletely disiippearcd on llic adilition n\' ;i, single drop of a solution 

 of alkaline sulphide, so that less than one dro[) completed the solution. 



(Jhemists, of course, regard cases of solution such as that under dis- 

 cussion as due to the genesis of soluble double salts, \Yhicli are loi'iucd 

 according- to ordinary laws of composition. The above experiments show 

 that this soluble double salt can be represented only by the formula Hg8, 

 2Na-S and that it is soluble in dilute caustic soda. 



The soluble mixture given by Mchu appears to be intended to repre- 

 sent the maximum solubility of mercuric sulphide, for he states that sulphy- 

 dric acid instantly produces a precipitate in it. As previously stated, it con- 

 tains two parts of crystallized simple sodie sulphide (Na'S, 9 IPO) to one 

 of HgS, which answers to lIgS + 2.07Na-S and is thus, so far as it goes, 

 confirmatory of the above experiments. 



Solubility of HgS in Na-s.—Tlic uiost carefully prepared solutions of sodium 

 sulphide dissolve mercuric sulphide freely. This statement is directly con- 

 trary to that which some of the chemists referred to have made, and it 

 would be a rash one if the evidence to be adduced for it depended simply 

 upon bringing solutions of sodic sulphide into contact with mercuric sul- 

 phide, for It is impossible to make certain that there is no trace of free caustic 

 alkali or of sulphydrate In solutions of Na=S, however closely Its analy- 

 sis may correspond to its theoretical composition. If, however, a solution 

 of sodic hydrate be treated with sulphydric acid, it Is gradually converted 

 into sodic sulphydrate and passes through a point at which the only com- 

 pound present is sodic protosnlphide. If mercuric sulphide be dissolved In 

 a mixture of sodic sulphide and sodic hydrate and the clear filtrate treated 

 with hydrogen sulphide, the mercuric sulphide begins to be precipitated 

 wlien very little free caustic alkali is left, and it Is continuously precipitated 

 until the entire amount of sodium present is converted Into the sulphydrate. 

 The purest preparations of sodic sulphide (Na'-S) which we have been able 

 to make, dissolve mercuric sulphide less freely than mixtures of sodic sul- 

 phide and sodic hydrate, but more freely than mixtures of sodic sulphide 

 and sodic sulphydrate. Different preparations, however, shown by most 

 careful analysis to correspond very accurately to the formula Xa'S, give 

 somewhat different results, possibly indicating a minute variation from ab- 



