144 Mechanical Assay of Qicartz, 



Other modifications of tlie assa_y, somewhat less operose, 

 are practised, but the method described gives unimpeachable 

 results, and from the sketch I have presented you will, 

 I trust, easily understand that this kind of assay requires 

 a certain amount of skill and knowledge, and therefore 

 involves expense, — expense which in many cases is prohibi- 

 tive of its use. 



Let us now contrast this method with the mechanical 

 assa}^ of quartz, and in the first place, let me suggest that 

 the ordinary treatment of quartz in the mill is itself a kind 

 of mechanical assay ; the quartz is pulverized under the 

 stamper, in a current of water, the lighter portion, — the 

 quartz particles, are carried forward in the stream, while the 

 gold by virtue of its greater specific gravity remains behind, 

 accumulating in the ripples of the mill. -The specific gravity 

 of quartz is 2.65, that of native gold of high quality may 

 be stated at 18.5, in other words the gold is seven times as 

 heavy as its own bulk of quartz. The associated minerals^ 

 the pyrites, &c., are of intermediate specific gravity, so that 

 these are also more or less liable to be retained with the 

 gold. There are impediments in the practice of quartz, 

 crushing preventing a perfect extraction. The quarts is 

 comparatively speaking only coarseh'" pulverized ; it is never 

 as fine as the meal of flour for instance. Some quartz is 

 mostly held back by the gold retained in the mill, and some 

 gold is carried away in the escaping quartz tailings, as a 

 result of this merely coarse pulverization. The pyrites 

 when separated is found to contain always some gold, at 

 times a not insignificant proportion of the whole contents ;, 

 for this reason, namely, that the gold and pyrites when 

 existing together in the stone are mostly in immediate 

 contact : and on rewashing the tailings, fragments may be 

 discovered, which, when examined under the microscope, are 

 seen to consist in part of gold, in part of pyrites, and 

 possibly also in part of quartz ; and these compound particles 

 will behave in accordance with the mean specific gravity 

 which they possess. There is a point in the pulverization 

 beyond which the practical treatment of quartz on the large 

 scale does not go, and this degree of pulverization does not 

 admit of a perfect mechanical separation of the gold. 



But although there is this limit in the treatment on the 

 large scale, there is no practical impediment in the way of 

 a more complete pulverization, when, for experiment, we 

 treat a mere handful of quartz. With a few appropriate 



