SPECIFIC GRAVITY 



15 



this is also the weight of the water displaced, that is, the weight of a volume of water 

 equal to the volume of the stone. Hence the specific gravity of the stone is, as. before. 



To take a numerical example. If the weight first added to balance the counterpoise be 



m = 10-784. grams (or other unit), and the weights which, together with the stone, balance 



the counterpoise be I = 4-803 grams, then the weight of the stone is ^ = ??j - Z = 10-784 - 



4-803 = 5-981 grams. Let the weight added when the stone is immersed in the water be 



t = 7-060. Then the loss of weight of the stone will be ^ - Z = 7-060 - 4-803 - 2-257 



, g 5-891 



grams, and a — J~_^i = g.^gy = 2-65. This is the specific gravity of rock-crystal (quartz). 



Other conditions being equal, the more delicate and sensitive the balance the more 

 accurate will be the specific gravity determination. Under favourable conditions and with 

 careful weighing the determination should be correct to the third place of decimals. 



Fig. 4. Bench for use in hydrostatic weighing with an ordinary balance. 



Certain precautions, however, must be attended ito. In the first place, all parts 

 immersed in water must be quite free from adhering air-bubbles ; these may often be 

 dislodged with the help of a clean platinum wire, but frequently it is advisable to bring 

 the water almost to the boiling-point while the stone and wire are immersed in it. For this 

 pui-pose the wire must be detached from the balance ; but before replacing the vessel of 

 water on the balance it should be allowed to cool, which process may be hastened by 

 immersing the vessel, still of course containing the wire and stone, in a bath of cold 

 water. Should the stone and wire be at all greasy through being handled, they will 

 not be properly wetted by the water, and should then be washed in alcohol, ether, 

 benzene, or a solution of soda ; in the latter case they must be afterwards rinsed in clean 

 water. Finally it must be borne in mind that the pesrcentage of error in the determina- 

 tion of a very small stone is greater than in that .of a larger one, since small errors in 

 weighing influence results more in the former case than in the latter. 



When the specific gravity of a number of stones is to be taken, a counterpoise is 

 chosen which exceeds the heaviest of them in weight. This may then remain throughout 

 the whole series of operations and the value of m be determined once for all, Two weighings 

 only are then necessary to find the value of I and t foj each stone. From I and t and the 

 constant value m the specific gravity can be calculated in each ease. 



The specific gravity of several fragments or small stones may be determined by placing 

 them together in the wire basket, shown in Fig, 2, and weighing them as a single stone. 



