IX.] CHEMISTRY OF NATURAL WATERS. 161 



When a double sulphide of sodium and hydrogen exists in 

 an alkaline water, it is possible, by boiling, to destroy the 

 compound, and by expelling the sulphide of hydrogen, to deter- 

 mine the amount of sulphur which is present as a fixed mono- 

 sulphide. But when the double sulphide has a base of mag- 

 nesium, or exists in a water containing an excess of a soluble 

 magnesian salt, the ready decomposition of sulphide of magne- 

 sium will cause the whole of the sulphur to be carried off by 

 boiling, in the form of sulphuretted hydrogen, with separation 

 of hydrate of magnesia, as is the case of the Bothwell water. 

 The following experiment was, however, devised, which shows 

 the existence of a double sulphide in this water, and at the 

 same time enables us to suggest a method which may probably 

 be used for the complete analysis of this and of similar waters. 



It is well known that solutions of alkaline and earthy sul- 

 phides dissolve tersulphide of arsenic, yielding double sul- 

 phides or sulpharsenites, whose formula, for the alkaline bases, 

 is, according to Berzelius, AsS 3 ,3MS, and for the earthy bases, 

 AsS 3 ,2MS. If these protosulphides are combined with sul- 

 phide of hydrogen, forming double salts, MS,HS, the latter 

 will be displaced by the arsenious sulphide. The presence of 

 such a compound in the Bothwell water was shown by adding 

 to it freshly precipitated and carefully washed tersulphide of 

 arsenic, which was rapidly dissolved, with an abundant disen- 

 gagement of sulphuretted hydrogen gas. The solution, after 

 digestion for a few minutes at 36 Centigrade, was filtered 

 from the excess of undissolved sulphide, and supersaturated 

 with acetic acid, which threw down a quantity of sulphide of 

 arsenic equal to .925 grammes to the litre. Another portion 

 of the same bottle of water, treated with an acid solution of 

 terchloride of arsenic, gave an amount of sulphide of arsenic 

 equal to 1.110 grammes to the litre. 



If now we suppose the dissolved sulphide of arsenic in the 

 experiment above described to have been in the state of a 

 sulpharsenite of magnesium, AsS 3 ,2MgS, in which the amount 

 of sulphur in the two terms is as 3:2, we should have 

 (3:2:: .925 : .617) .617 grammes of the sulphide of arsenic 



