EFFECT OF FOREIGN SUBSTANCES. 431 



drate and sodic liydrate, containing- very little of the latter, was suddenly 

 diluted with cold water to two hundred times its volume and rapidly filtered. 

 Minute globules of mercury could be seen with the black sulphide on the 

 filter. On digestion (after thorough washing) with very dilute nitric acid, 

 a solution was obtained from which sulphydric acid preci[)itated black sul- 

 phide. The decomposition thus appears to be the same in solutions of each 

 of the compounds HgS, K"S and IlgS, 4Na-S. 



Influence of foreign substances. — Tlic fiict that sodium carbouatcs do uot pre- 

 vent the solution of mercuric sulphide is evident both from Mchu's residt 

 and from our own. As was nidUtioned above, mercuric sulphide dissolves 

 abundantly in a solution containing these carbonates and sodic sulphides. 

 The chief constituents of tlie waters of Steanaboat Springs and Sulphur 

 Bank, besides alkaline carbonates and sulphides, are borax and salt. Ex- 

 periments show that borax solutions precipitate a portion of the mercury 

 from solution, but not the whole. The precipitation does not appear to bo 

 progressive, like that accompanying dilution, but to reach a sharp limit 

 beyond which further additions produce no effect. A large amount of bo- 

 rax added to a concentrated solution of sodic sulphide and sodic sulphy- 

 drate does not rob it of the power to dissolve mercuric sulphide. 



It is easy to imagine reactions by which borax ma}^ precipitate a por- 

 tion of the mercuric sulphide. It seems possible, for example, that neutral 

 borate is formed at the expense of the sodic sulphide combined with the 

 mercuric sulphide. The sodic sulphide would then be converted to sulphy- 

 drate and mercuric sulphide would precipitate. But the behavior of solu- 

 tions of I)orax to sulphydric acid and to alkaline sulphides is very peculiar, 

 and, so far as I am aware, has not been thoroughly investigated.' Very 

 concentrated solutions of sodium chloride do not precipitate mercuric sul- 

 pliide from strong solutions in mixtures of sodic sulphide and sulphydrate, 

 and they even appear to delay, but not to prevent, precipitation by dilution. 



The waters of Steamboat Springs contain no ammonia and probably 

 no organic matter. Those of Sulphur Bank carry ammonia, and all of the 

 mines examined in California show more or less organic matter. It is highh- 

 probable, therefore, that during the period of ore-deposition more or less 



' Gmelin-Kiant, loc. cit., vol. 2, p. 160. 



