TECHNICAL CHEMISTRY. 341 



selves, to whom now it is entirely lost, but also for the sake 

 of diminishing the nuisance to the neighbourhood, is patent 

 to all. The waste, even when carefully stamped down and 

 when the surface is made as hard as possible or is even 

 covered with cinders, invariably undergoes a certain amount 

 of oxidation. The insoluble mono-sulphide of calcium then 

 becomes converted into a soluble higher sulphide, and this 

 coming in contact with rain or drainage water dissolves, 

 and the solution finds its way into the ordinary drains or 

 streams of a district. There meeting with the carbonic 

 acid of the air, or with the acid discharges from the 

 chemical works, this yellow liquor evolves sulphuretted 

 hydrogen sufficient to become a nuisance to the inhabitants 

 of districts lying even several miles away from the waste. 



Many proposals have been made for recovering the sulphur 

 from the waste. Amongst these, that patented in England 

 by Mr. Ludwig Mond is the most important, and has proved 

 the most successful. This process depends upon the fact that 

 the newly lixiviated waste, being porous and warm, under- 

 goes oxidation when exposed to the action of air with forma- 

 tion of soluble sulphur compounds, so that on treating the 

 oxidized waste with water not less than half of the total 

 sulphur is obtained in solution in such a condition that, upon 

 acidification with hydrochloric acid, the whole of the sulphur 

 in combination is precipitated in the form of a finely-divided 

 yellow powder, some fine samples of which you here see. 

 The form in which the sulphur dissolves is chiefly as hypo- 

 sulphite and disulphide of calcium, and these must be 

 present in such proportions that on treatment with acid 

 two molecules of sulphuretted hydrogen are evolved to 

 one molecule of sulphurous acid ; and hence sulphur alone 

 is precipitated without evolution of either of the two gases ; 

 thus : 



As much as 50 per cent, of the sulphur may, according to 

 Mr. Mond, be thus recovered, but as a rule not more than 

 30 per cent, is practically obtained. The waste from which 

 this fraction of the sulphur has thus been extracted is now 

 a much more innocuous body than before, and the manufac- 

 turer is a gainer by the production of the sulphur. Still as 



