142 Mechanical Assay of QuaHz. 



operated on ; while for anything- finer than such coarse gold, 

 we depend on the assay of a fair sample of the pulverized 

 specimen. We weigh out- a sample of the pulverized sieved 

 stone ; in choosing the amount we are about to operate upon, 

 we find it convenient to take a definite fraction of a ton : 

 say, a five-thousandth part of a ton, which is 3136 grains. 

 Employing this quantity, we can easily refer our result to 

 the ton of stone, by merely multiplying the weight of gold 

 obtained by 5,000. We take this -jj^^ of a ton, and roast it 

 in a cast iron or shallow earthenware tray, in a muffle or 

 oven, at a dull red heat. The iron pyrites and other com- 

 pounds of sulphur and arsenic, &c., in the quartz are 

 inimical to the extraction of the gold, and one of the 

 ways of eliminating them is this process of roasting ; we 

 burn out the sulphur and oxidize the iron with which 

 it is combined at so low a temperature as to avoid melting 

 any of the constituents of our sample, and this prepares us 

 for the next operation, the fusion. 



I will endeavour to convey an idea of this fusion process 

 (which in reality involves something more than mere fusion) 

 by reference to what takes place during the manufacture of 

 flint glass. You know that quartz sand forms the basis of 

 ordinary flint glass ; that in the manufacture the quartz 

 sand, or silica, is used with oxide of lead and with alkali, 

 and that by the combination of these the glass is produced. 

 It is just so in the fusion of a quartz assay ; oxide of lead 

 and alkali (carbonate of soda for instance) are added so as 

 to form a fusible compound with the quartz, so indeed 

 that the quartz is wholly dissolved. If quartz containing 

 gold were thus converted into a molten glass, it mio'ht be 

 presumed that the heavy gold would gravitate through the 

 iDath of lighter molten glassy material, and so it would 

 if the gold happened to exist in the form of coarse grains or 

 nuggets ; but very fine particles of gold do not very readily 

 collect together at the bottom of such a bath, and therefore 

 other means for the collection of the gold are involved in 

 this fusion process. 



If charcoal be heated with oxide of lead, the charcoal is 

 consumed, and metallic lead is revived from the oxide. One 

 part of pure carbon will thus revive thirty-four and a half 

 times its weight of lead from the protoxide. Nitre on the 

 other hand oxidizes about five times its weight of metallic 

 lead, forming oxide of lead as the result of this change. In 

 the opposing action of these two materials, the reducing 



