dust, evaporation, etc., in suitable stoppered and capped bottles or short wide tubes in the case of A and B. A number of weighted glass bulbs or a series of small mineral fragments of ascertained specific gravity are very useful as "indicators." It is a good plan to keep one or more of these indicators in each liquid to be employed. To avoid doubt and confusion these indicators, whether bulbs or mineral fragments, should present so characteristic a form or colour or marking that their identity and value can be recognized at once. It is worth while adding the remark that liquids A, B, and C, are required much less frequently than the less dense liquids, and that when the position of a doubtful stone has been once fixed by the density test so as to prove that it belongs to a particular group, then it may be necessary to call in the aid of the dichroiscope and of the scale of hardness in order to learn to what species in that group the stone really belongs.
2. By weighing a stone in air and then in some liquid of known density, the weight of the bulk of the latter displaced by the stone is ascertained. If, for example, a sapphire weighing 4 grains in air weighs but 3 grains in water, it has evidently displaced 1 grain of water, becoming lighter by that amount. So the number 4 represents the specific gravity of sapphire, showing, as it does, the number of times that the weight of any bulk of that stone contains the weight of an equal bulk of water. An example of an actual experiment of this kind will serve to illustrate this, the ordinary method of taking specific gravities, better than any further explanation of the principle involved.
| A yellow sapphire weighed in air | 12.896 | grams. |
| A yellow sapphire„weighed in water„ | 9.677 | grams.„ |
| ——— | ||
| Difference, that is, weight of water displaced | 3.219 | grams.„ |
The proportion will be :
| Wt. water displaced. | Sp. gr. water. | Wt. sapphire. | Sp. gr. sapphire. |
| 3.219 : | 1 = | 12.896 | : x |
| x.= 4-006 |
