195. The quantity of sulphur required for the precipitation of silver is really only as 16 to 108, but a great waste occurs in the process described, from two causes: Firstly, there is always a certain quantity of base metal dissolved with the silver, which also takes its portion of sulphur from the calcium sulphide; secondly, a large proportion of the sulphur is thrown down in a free state, and, in the usual course of procedure, is totally wasted by being burned off in the roasting of the precipitate.

The reason for this precipitation of free sulphur is that the precipitant is necessarily a pentasulphide, for the calcium mono sulphide is insoluble, and the bisulphide is only soluble with heat (vide Regnault), so that for our purpose we are restricted to the pentasulphide.

But in the precipitation, only one-fifth part of the sulphur of the pentasulphide can enter into combination with the silver, forming silver sulphide, and as the calcium combines with the oxygen and hyposulphurous acid which were combined with the silver, the remaining four-fifths of the sulphur are unoccupied and useless.

Base metals, when present in the solution, act similarly, combining with only a portion of the sulphur of the pentasulphide, only arsenic and antimony forming pentasulphides. Gold forms tersulphide, but its quantity is usually insignificant in the lixivium.