FORMATION OF MEKOURIC SULPUOSALT. 429 



doubt that chemists have observed it, and that, in consequence of this ob- 

 servation, the general statement of tlie solubility of mercuric sulphide in 

 alkaline sulphydrates has remained in chemical literature in spite of the 

 observations of Weber and Barfoed. The ])reparation in which I origi- 

 nally observed this important reaction was one from which mercury had 

 already been removed by precipitation with hydrosulphui-ic acid The ex- 

 periment was afterwards repeated by Dr. Melville with several preparations 

 of sulphydrate which had I)een accurately analyzed and had been tested 

 in numerous ways. 



Now, in a mixture of the carbonates and sulphides at the temperature 

 of the water-bath, either sodic sulphide or sulphydrate is present, or, more 

 probably, they coexist. If, then, mercuric sulphide be added to such a so- 

 lution, either sodic sulphide combines directly with mercuric sulphide or 

 sodic sulphydrate is decomposed by mei-curic sulphide, setting free liydro- 

 o-en sulphide, which must be immediately absorbed by sodium monocar- 

 bonate. Hence, in any case the salt dissolved in the solvent must be of 

 the form HgS, »Na-8. 



Effects of dilution. — Laborator}' experiments are usually made with solutions 

 which are more concentrated than those found in nature. Hence the effect 

 of dilutions on solutions of HgS, ^Na^S is important. Whether mercuric 

 sulphide be dissolved in a mixture of sodium protosulphide and sodium hy- 

 drate or of the former and sulphydrate, dilution with cold water precipi- 

 tates mercuric sulphide. The process is gradual, yet progresses in stages. 

 Thus a mixture of solutions of sodic sulphide and sodic hydrate of a vol- 

 ume of 3.9cm^ was nearly saturated with mercuric sulphide. The mixt- 

 ure was represented by the formula HgS + 2.04Na-S-|-1.38NaHO + aq 

 and contained 0.3349 gram of mercuric sulphide. Supposing no change 

 of volume to have taken place, this is equivalent to 8G grams of mercuric 

 sulphide per liter. On dilution no precipitate was observable until 25cnr 

 of water had been added, or until the contents were reduced to 11. (i 

 grams of mercuric sulphide per liter. Precipitation appeared to continue 

 until about lOOcm' of water had been added. There then remained in 

 solution 0.0753 gram by weight, or 0.724 gram per liter. Tlie filtrate 

 remained clear until enough water had been added to reduce the strength 



