This result gives the weight of a bulk of water equal to that of the specimen, and by dividing the weight of the specimen in air by this number, the specific gravity is obtained.
| Specific gravity = | weight of substance in air |
| weight of equal bulk of water |
If, however, the substance is in the form of fine sand, or very small lumps, it is better, after weighing it carefully, to take a small dry phial furnished with a stopper; counterpoise this phial accurately in the weight-scale by shot or strips of lead, then fill it completely with pure water, taking care that no bubbles of air are left in, and weigh the quantity of water it contains: afterwards empty the bottle and dry it inside.
Next fill the bottle about two-thirds full of the powder to be examined, weigh this and record the weight. Then fill the bottle once more with water, taking care, as before, that all bubbles are expelled and none of the powder washed out. Once more weigh it.
We have then to make the following calculation:
| Weight of powder and water in grains | = | |
| Deduct weight of powder alone | = | |
| Difference (weight of water left in bottle) | = | |
| Weight of bottle full of water in grains | = | |
| Weight of water left in bottle | = | |
| Difference (weight of water displaced by, and equal in bulk to, powder) | = |
| The specific gravity = | weight of powder in air |
| 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 | 11/5 |
| Platinum | 191/2 - 22 not hammered. |
| Iridium | 187/10 |
| Gold | 153/4 - 191/4 ditto |
| Mercury | 131/2 |
| Palladium | 117/10 |
| Lead | 111/4 |
| Rhodium | 103/5 |
| Silver | 10 |
| Copper | 73/4 - 8 |
| Brass | 81/2 |
| Lead ore (galena) | 71/2 |
| Copper pyrites | 5 |
| Iron pyrites | 4 |
| Diamond | 31/2 |
| Sand | 23/5 - 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.