If the specimen then is large enough to be suspended conveniently by a thread, weigh it first in air by a fine balance, expressing the result in grains, and taking care previously to remove dust or loosely adhering particles. Then suspend it by a horsehair from the scale-pan (it is convenient to have a hook attached to it for this purpose,) and thus suspended, immerse it and re-weigh it in water, taking care that it is covered on all sides by at least half an inch of water, and carefully brushing off with a feather any bubbles of air that adhere to the surface. The results may then be noted as follows:-- Weight of substance in the air in grains Deduct weight of ditto in water -------- Difference 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. weight of substance in air Specific gravity --------------------------------- 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) } ------- 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:--