SPECIFIC GRAVITY 21 



its specific gravity is identical with that of the liquid, which is about 3-i3 at ordinary tempera- 

 tures. In the third case, if the stone should rise to the surface, even after being pressed 

 down with a glass rod, we know that its specific gravity is less than that of the liquid. 

 When this happens, benzene is added drop by drop and the mixture well stirred. The 

 addition of benzene is cautiously continued until a mixture of such a density is an-ived at that 



Fig. 7. Westphal's balance for determining the specific gravity of liquids. (Half actual size.) 



the stone no longer rises to the surface, but remains suspended in the liquid, moving neither 

 up nor down. We then know that the mixture has the same specific gravity as the 

 stone. 



The value of the specific gravity of the mixture of methylene iodide and benzene, and 

 consequently that of the stone, has now to be determined. This may be done by means of 

 the pycnoraeter, which is weighed first full of water, and then full of the mixture. 

 Deducting from each of these weights the weight of the pycnometer itself, Ihe weights of 

 equal volumes of water and of the mixture are known ; and dividing the latter by the former 

 the specific gravity of the mixture is found. 



The specific gravity of a liquid is determined more conveniently, and with sufficient 

 accuracy for practical purposes, by means of Westphal's balance. This instrument, together 

 with the accessories necessary for determining the specific gravity of a liquid, is shown in 

 Fig. 7. The small scale-pan previously used (Fig. 5) is now. replaced by a glass float, q. 



