SODIUM SULPHIDE IN NATUltAL WATERS. 427 



neutral carbdiiatoand the sesciuicarhonato areknown to crystallize from many 

 natural waters. It is even ditlicult to produce the acid carbonate free from 

 the neutral salt, and the acid carbonate of conniierce, tliouoh designed to Ije 

 pure, invariably contains a considerable amount of the more basic compound. 

 Hot solutions of acid carbonate lose carbonic acid rapidly and cold solu- 

 tions evaporating in dry air also lose a large part of their acidity, so tiiat 

 the neutral carbonate and carljon dioxide may coexist. In my opinion it is 

 onlvsafe to regard natural waters as in general containing Ijoth carbonates. 



When hydrogen sulphide is passed through waters containing neutral 

 carbonate at ordinary temperatures the following reaction is known to take 

 place: Na=CO"+rr'S=zNaIICO' + NaIIS ; so that, if the solution of the 

 neutral carbonate be moderately strong, a portion of the less soluble acid 

 carbonate is precipitated by hydrogen sulphide. If hydrogen sulphide be 

 passed tlirough the solution until it is only semi-saturated or if a saturated 

 solution be added to a solution of the neutral carbonate, the composition 

 will be Na^COHNaHCO'+NaHS. 



It is evidently conceivable that the neutral carbonate should react 

 upon the sulphydrate, producing sodium sulphide and acid carbonate. 

 This reaction cannot take place under ordinary conditions, however, for the 

 thermal effect of Na=CO''-f-NaHS = NaHCO=' + Na=S is negative. It does 

 not follow that this reaction may not take place at temperatures approach- 

 ing 100°. Indeed, in connection with the known fiicts as to the solubility 

 and hydration of sodium carbonate at different temperatures, it is a conse- 

 quence of a somewhat complex train of reasoning on the thermal effects of 

 the formation of the compounds involved that the following reaction must 

 give a positive thermal effect when the temperature exceeds 80°: 

 2Na-CO^ + 2NaHC0^4-2NaHS = 2NaHCO\ ' 

 Na- CO' + NaHCO' + NaHS -f Na^S. 

 If this reaction actually takes place, a mixture of the two carbonates with 

 the sulphydrate, raised to a temperature of above 80°, yields a portion of the 

 simple sulphide of sodium. This appears to give a greater thermal effect 

 than any other reaction which can be devised between the ingredients. It 

 does not of necessity follow that it takes place ; for the salts may possibly 

 present unknown resistances to combination similar to the resistance which 



