SPECIFIC GRAVITY 



13 



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To avoid serious error in determinations of specific gravity the presence of bubbles of 

 air must be carefully guarded against. These often cling with great pertinacity to the stone, 

 but may usually be dislodged with the help of a clean platinum wire or by gently warming 

 the flask. Should any collect under the stopper, the flask must be emptied and refilled with 

 distilled watei;. 



When all the precautions mentioned above are carefully attended to, the accuracy of 

 the determination varies with the delicacy of the balance. With a good balance and a little 

 practice the pycnometer method is capable of giving results accurate to the third place of 

 decimals. It has the further advantage of enabling the operator to determine 

 the specific gravity of several small stones or fragments taken together. The 

 application of this method of determination is limited in two directions : it is 

 useless, on the one hand, for determining the specific gravity of stones too 

 large to pass through the neck of the bottle, and, on the other hand, is not 

 sufficiently accurate when only very small quantities of material are available. 



2. Method with the Hydrostatic Balance. — This method is more frequently 

 used, and with careful manipulation is perhaps quite as accurate as the method 

 just described. It depends on the fact that a body when immersed in water 

 weighs less than when in air. According to the well-known principle of 

 Archimedes, the loss in weight is equal to the weight of the water displaced, 

 that is, of a volume of water equal to the volume of the body. 



The stone is first weighed in air; let this weight be g. It is then 

 suspended from the arm of the balance by a hair, fine wire, or thread, and 

 weighed in water ; let this weight he J". It is obvious that the difference in 

 weight, ^ — ^, is the weight of the water displaced, that is, of a volume of 

 water equal to the volume of the stone. The specific gravity, d, of the stone 



JO- 



is therefore given by d = — ZTT"' 



The hydrostatic balance used in the above operation differs in no 

 essential respect from an ordinary balance. It is merely arranged so that 

 the right-hand scale-pan hangs from a much shorter support than the 

 other. Frequently the right-hand scale-pan of an ordinary balance can be 

 replaced by one with shorter supports and a small hook on the under side. 

 When the stone is immersed in water, the thread or wire which carries it is 

 attached to this hook. For the purpose of conveniently holding the stone, 

 the lower end of the thread or wire may carry a small clip, as shown in 

 Fig. 5, or the wire may be simply wound round the stone so that it rests in 

 a stirrup or in a small spiral. A basket of this description is shown in Fig. 2. 

 The water is placed in a glass vessel under the right-hand scale-pan with 

 the short support, and the wire carrying the clip or other arrangement is 

 immersed in water during the whole operation, so that it is weighed in water even before 

 the stone has been fixed in position for the second weighing. It is then unnecessary 

 to specially determine the weight of the wire with its attachment and the loss of weight 

 of this in water, as the effect in one weighing is cancelled by that in the other. 



The use of a balance with specially arranged scale-pans, as described above, is by no 

 means essential, however, for the determination of specific gravity. Every jeweller has a 

 good balance with scale-pans having supports of equal length, and this can easily be used as 

 a hydrostatic balance, as illustrated in Fig. 8. For this purpose, the vessel containing water 

 is placed on a small table or bench, such as is shown in Figs. 3 and 4, which stands over 



Fig. 2. Basket 

 of platinum 

 wire for holding 

 stone in deter- 

 mination of 

 specific gravity. 



