3. The third method of taking specific gravities does not admit of great exactness. A small wide-mouth bottle or beaker, with a ground rim and ground glass cover, both truly plane, is filled with water, the cover placed in position, avoiding air-bubbles and wiping off any water outside the vessel, and then weighing it and its contents. Let this weight be x. Now introduce the gem and replace the lid as before; let the present weight be y, and that of the gem in air w; then approximately,
In employing this method the vessel used should be no larger than will contain the specimen.
Specific gravities may be ascertained by means of contrivances dependent upon the measurement of the liquid the objects displace from a vessel of known capacity or carefully graduated. The space at our disposal will not allow of any further details on this subject. But a caution as to the necessity in all specific gravity experiments of getting rid of air-bubbles may not be out of place. To attain this end boiled water should be used, and if mechanical contrivances fail (a feather or sable pencil) then the liquid and stone should be placed under the receiver of an air-pump and the air exhausted.
Details concerning the specific gravity of each kind of precious stone will be found in chapter vii. The following table gives a fair number of average densities arranged in regular sequence:
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FOUR AND ABOVE,
Hæmatite 5·3
Pyrites 5·2
Zircon 4·6
Almandine 4·2
Sapphire 4·0
THREE AND ABOVE.
Green garnet 3·85
Chrysoberyl 3·76
Pyrope 3·75
Hessonite 3·66
Spinel 3·65
Topaz 3·55
Diamond 3·52
Peridot 3·38
Spodumene 3·20
Tourmaline 3·10
TWO AND ABOVE.
Phenakite 2·98
Turquoise 2·75
Labradorite 2·72
Beryl 2·70
Amethyst 2·66
Rock crystal 2·65
Iolite 2·63
Moonstone 2·58
Opal 2·20
