weight of powder in air The specific gravity = -------------------------- weight of water displaced.
It may be useful to know the specific gravity of various substances at all resembling gold in weight or appearance, and we therefore append the following short table. The specific gravity of water is assumed to be unity:--
Osmium 21-1/5 Platinum 19-1/2 - 22 not hammered. Iridium 18-7/10 Gold 15-3/4 - 19-1/4 ditto Mercury 13-1/2 Palladium 11-7/10 Lead 11-1/4 Rhodium 10-3/5 Silver 10 Copper 7-3/4 - 8 Brass 8-1/2 Lead ore (galena) 7-1/2 Copper pyrites 5 Iron pyrites 4 Diamond 3-1/2 Sand 2-3/5 - 3
By the help of this table the value[18] of auriferous sand may also be in some degree estimated, since, as will be seen, the specific gravity of most of the sands is under 3, while that of the most impure gold is 12; so that if the specific gravity of the sands themselves, when experimented on, is much greater than that of ordinary sand, it is likely that the excess will be for the most part gold, in a district otherwise known to be auriferous: the greater the specific gravity, too, the greater probability there is, of this being the cause. It may also be worth while to mention here, that the specific gravity of those pepitas or lumps of gold which present a fine yellow color varies generally from 14-7/10 to 18-8/10; but when much paler they may range as low as 12-1/2, which is that of a mineral called _electrum_, which will be described presently, and which is a mixture of silver and gold.
When a piece of gold is broken (which is not done without difficulty--greater in proportion to its purity,) the fractured edges are very uneven and torn, exhibiting a peculiar fibrous appearance, known to mineralogists as "fine hackly." This fracture indicates that the mineral is _torn asunder_ and not really broken, and is a proof of considerable toughness.
The form in which gold is found is various. It is sometimes crystalline, in eight or twelve-sided regular figures, passing into cubes, but the crystals are generally small and rare. In case of such crystals being found, it is well worth knowing that they possess a value as mineral specimens far beyond that of the gold which they contain.
More frequently the metal is found in lumps or grains, called by the Spaniards _pepitas_, varying in size from that of a pin's head to masses weighing, as has been already mentioned, nearly one hundred pounds troy. The term _pepita_ is only applied to grains of some magnitude, and the most common limits of size are from that of a small pin's head to that of a nut or gooseberry.