14 



GENERAL CHARACTERS OF PRECIOUS STONES 



the scale-pan of the balance, leaving the latter quite free to move. The thread or wire with 

 the attachment for can-ying the stone is fastened to the hook on the upper part of the 

 right-hand scale-pan. Obviously the water-containing vessel must not be so large as to 

 interfere with the movement of the balance, and the length of the wire must be such that 



Fig. 3. Oi-dinary balance with arrangement for hydrostatic weighing. 



the stone it carries will not touch the bottom of the vessel nor rise partly out of the water 

 when the balance is swinging. 



In the determination of a specific gravity with such a balance the best mode of pro- 

 cedure is as follows : A fragment of a mineral or metal or any object, the weight of which 

 is greater than that of the stone whose specific gravity is to be determined, is placed in the 

 left-hand scale-pan. This acts as a counterpoise, and must remain throughout the whole 

 operation. It has been pointed out above, that the wire with its carrying attachment must 

 I'emain immersed in water till the completion of the operation. Weights are then placed in 

 the right-hand scale-pan so as to balance the counterpoise ; let the weight required be m. 

 These weights are then replaced by the stone ; equilibrium is restored by adding a weight, /, 

 in the right-hand scale-pan. The weight of the stone will therefore he m — I = ff say. The 

 stone is then attached to the end of the wire and immersed in water, care being taken that it 

 hangs freely without touching the sides and bottom of the vessel. By adding more weights 

 to the right-hand scale-pan the balance is again brought into equilibrium. If the total 

 weight is now t, then, the loss in weight of the stone, when weighed inwater, is t — I, and 



