22 GENERAL CHARACTEKS OF PRECIOUS STONES 



containing a thermometer. This hangs by a fine wire from the hook c, and is sufficiently 

 heavy to balance the counterpoise and bring the pointer to zero. If, however, the pointer 

 should not stand exactly at zero, it may be brought to this position by turning the levelling 

 screw m. When the float is immersed in a vessel containing distilled water, its loss in 

 weight is equal to the weight of the displaced water. The pointer is brought to zero again 

 by placing a unit weight, N, on the hook, c, the sizes of the float and the weights having 

 been so arranged that this shall be the case. The loss of weight in the float is, therefoi'e, 

 represented by IN. Any other liquid which requires a unit weight, N, to bring the beam 

 again into the horizontal position will have the same specific gravity as water, that is, 

 a specific gravity of 1. Should twice or thrice the unit weight, 2N or SN, be required, 

 then the specific gravity of the liquid is 2 or 3, that is, it is two or three times as heavy 

 as water. For intermediate values, the riders must be used in the manner explained 

 previously. Thus, for example, let us suppose the float immersed in some liquid, which to 

 bring the beam into the horizontal position requires 3A^ at c, N at the second division, n 

 ( = T^^ at the fifth division, and n^ ( = roiriV) at the ninth division. Then the specific 

 gravity of the liquid is given directly by the reading 3'259. The specific gravity of the 

 liquid contained in the vessel in Fig. 7 is, in the same way, 2-707. Hence the specific 

 gravity is given directly by the readings, and there is no necessity for the smallest 

 calculation. 



With this instrument it is possible with a little practice to determine the specific 

 gravity of a liquid correct to the second place of decimals, and the whole operation can be 

 performed in a few minutes. All that is necessary is to immerse the float in the narrow 

 glass cylinder containing the liquid, and by the addition of weights to bring the beam into 

 the horizontal position. 



The use of indicators enables the determination of the specific gravity of a liquid to 

 be performed with still greater rapidity ; though the values obtained by this method are 

 only approximate, they are sufficient for all practical purposes connected with the determina- 

 tion of precious stones. There are used as indicators either small, differently weighted glass 

 bulbs, known as specific gravity beads, or small mineral fragments of different specific 

 gravities, ranging by small amounts from the specific gravity of the lightest to that of the 

 heaviest of precious stones. The following minerals, among many others, may be selected 

 for such a series of indicators : chalcedony (sp. gr. = 2'560), microcline (sp. gr. = 2'591), 

 petalite (sp. gr. = 2-648), labradorite (sp. gr. = 2-686), calcite (sp. gr. = 2-728), &c., this 

 being a portion of a series of indicators suitable for the use under discussion. 



Such a series would be used in the following manner : When the liquid has been so 

 diluted that its specific gravity is identical with that of the stone to be determined, the 

 lightest of the series of indicators, chalcedony, say, is put into it. Should it float it is 

 taken out (and should be washed with benzene), and each of the series tried in succession 

 until one is found which sinks to the bottom of the vessel. To take a special case, let us 

 suppose that petalite floats while labradorite sinks. We should then know that the specific 

 gravity of the liquid, and therefore of the stone under examination, lies between 2-648 and 

 2-686. There would then be a probability, or at least a possibility, that the stone is quartz 

 (rock-crystal, amethyst, &c.), which has a specific gravity of 2-65. In practice, a set of 

 mineral fragments, the specific gravity of each of which has been accurately determined, 

 should be kept solely for use as indicators, and it should be possible to readily distin- 

 guish one fragment from another. 



The method of determining specific gravity by the aid of heavy liquids, and specially 

 by that of methylene iodide, has the advantage of giving results which are quickly and 



