424 QUICKSILVER DEPOSITS OE THE PACIFIC SLOPE. 



solute purity. It does not seem a priori inipi'obable that the soluble salt 

 wlien the sodic sulphide is absolutely pure is HgS, 3Na^S, and one of our 

 preparations gave almost exactly this result. It may also be that the mixt- 

 ures of HgS, 2Na-S and HgS, 4Na-S are formed in proportions varying 

 with other conditions than the purity of the sodium sulphide, such as tem- 

 perature and concentration. 



Insolubility of HgS in cold NaHs. — Repeated experimeuts and analyses under- 

 taken during this investigation have shown that mercuric sulphide is totally 

 insoluble in sodic sulphydrate at ordinary temperatures and that any prep- 

 aration of this compound which will dissolve a trace of mercuric sulphide 

 can be shown by analysis lo fall short of complete saturation. Along time 

 and an enormous quantity of hydrogen sulphide are required to completely 

 saturate even a small amount of caustic soda with sulphur. As already 

 mentioned, both AVeber and Barfoed were aware of the insolubility of mer- 

 curic sulphide in sodic sulphydrate at ordinary temperatures. It will be 

 seen later that the behavior of these compounds varies with the tempera- 

 ture. If mercuric sulphide be left in contact with cold sodium sulphydrate 

 for twenty -four hours, just a trace of mercury goes into the solution. This 

 is due to the spontaneous loss of hydrogen sulphide which the sulphydrate 

 is well known to undergo. 



The absolute want of power of a preparation of sodium sulphydrate 

 to dissolve a trace of mercuric sulphide is perhaps the best known test of 

 its freedom from the alkaline protosulphide. This test does not show the 

 absence of pol}sulphides, however, for we have frequently found mercm-ic 

 sulphide totally insoluble in solutions of sodic sulphydrate which possessed 

 a yellow color and which were proved l)y analysis to contain an excess of 

 sulphur. This corresponds to Barfoed's observation. The occurrence of 

 alkaline polysulphides in nature, excepting near the surface of the earth, 

 seems so improbable that I have undertaken no investigations of the con- 

 ditions under which they dissolve mercuric sulphide. 



Solubility of HgS in mixtures of NaS and Na'S, H'S.— For tllC pUrpOSC of determining 



the character of solutions of mercuric sulphide in mixtures of sodium sul- 

 phide and sulphydrate, clear solutions of mercuric sulphide in sodium sul- 

 phide and sodium hydrate were made, all of the reagents being carefully 



