SPECIFIC GRAVITY 



19 



(3) Finally the stone is fixed in the clip p and immersed in the water ; the weight 

 now required to restore equilibrium was 3-97857V^, The loss of weight of the stone was* 

 therefore (3-9785 - 3-«12)iV = 0-1665iV, and the specific gravity 



0*443 

 d = r..-,f^nf- = 2"66. This is again the specific gravity of rock- 

 crystal (quartz), and the stone may therefore be identified as quartz. 



This method of determination enables the specific gravity of 

 a stone to be found correctly to two places of decimals with very 

 little trouble, provided that the precautions already mentioned 

 have been observed : namely, that the stone is free from air-bubbles, 

 that it does not come in contact with the sides of the vessel nor rise 

 out of the water, and that it is not too small. The specific gravity 

 of a stone weighing half a carat, that is about one-tenth of a gram, 

 can be determined accurately to the second place of decimals ; though 

 the figure in the second place will be uncertain when the stone only 

 weighs ;| or I- of a carat, the determination is still useful for practical 

 purposes. With stones smaller than this, however, the results are 

 not sufficiently reliable. If supported in the basket shown in Fig. 2, 

 or in a net made of platinum gauze fixed to the clip, a number of 

 such small stones may be weighed together. 



In this method, as previously, the first weighing, giving the 

 constant for the counterpoise of the instrument and the particular 

 wire and clip to be used, may be performed once for all, so that 

 afterwards only two weighings are necessary, and this effects a 

 considerable saving in time. Should the stone, whose specific 

 gravity is to be determined, be so heavy that it raises the counter- 

 poise by its own weight, then suitable additions must be made to 

 the weight of the latter, but in this case the balance will lose in 

 sensitiv(;ness. Usually, however, the weight of the counterpoise is 

 sufficient for most of the purposes for which the instrument is 

 intended. 



(4) Method zvith Jolly's Spring-halaiice. — The spring-balance 

 invented by, and named after. Jolly, formerly a physicist in Munich, 

 possesses considerable advantages, for by its means the specific gravity 

 of stones of fair size can be determined with sufficient accuracy and 

 very simple manipulation, no weights being required. The coi;- 

 struction of the instrument is shown in Fig. 6. A vertical rectangular 

 support, acd, about a yard and a half in length, stands on a base b 

 furnished with levelling screws. The vertical support carries on one 

 face a strip of plane mirrtn- on which a scale is engraved. IVom a 

 horizontal projection at the upper end, a, of the vertical support 

 hangs a spiral of fine steel wire. This carries at its lower end a fine 

 platinum wire, to which are attached, one above the other, two small cups, m and in 

 (Fig. 6, A), of glass or platinum gauze. The length of wire lying between the two cups 

 also bears two reference marks at o and o. The lower cup m' is immersed in water 

 contained in the vessel ff which rests on the stand h. This stand, h, can be moved 

 up and down the vertical support and fixed in any desired position. 



In using the instrument, which for the first reading must remain unloaded, the lower 



KiG. 6. Jolly's Spring- 

 balance for determining 

 specific gravity. 



