Chap. XIX.] 



HYDROMETERS. 



201 



the volume of water in c.c. displaced by the solid 

 body. But each c.c. =1 gramme, and, therefore, one 

 obtains at once the weight of water displaced equal 

 to the volume of the solid body, and the sp. gr. 

 of the body is at once ascertained by dividing its 

 weight in air by the weight of the displaced volume 

 of liquid. If the body is soluble in water, take some 

 liquid in which it is not soluble, and put this liquid 

 into the tube. Then proceed as before. The volume 

 of the displaced liquid is the same as that of water 

 would be. Therefore, the number of c.c. displaced 

 gives at once the weight in grammes of a volume of 

 water equal to the volume of the solid body, and the 

 calculation may be completed at once. The method 

 is applicable to any solid, if only 

 the tubes be filled with a liquid 

 in which the body is insoluble. 



Hydrometers, or areo- 

 meters, are instruments designed 

 for readily indicating the specific 

 gravity of a body. Nicholson's 

 hydrometer is shown in Fig. 97. 



It consists of a hollow metallic 

 cylinder AB, which is made to float 



/ 



by the weight of an attached cone 



EF. The cylinder carries at its 



upper end a thin stem which bears 



a metallic disc CD. The instrument 



is immersed in water, and weights 



are placed on the disc sufficient to 



bring the hydrometer down in the 



water to the level of a mark o on 



the stem. The body whose specific 



gravity is to be determined is now placed on the disc. 



Its weight brings the hydrometer lower in the water ; 



weights are, therefore, taken off till the instrument is 



at its former level. The weights removed give the 



Fig, 97. Nicholson's 

 Hydrometer. 



