ABSOLUTE AND RELATIVE SPECIFIC GRAVITY. 39 



different weights. A piece of lead is much heavier than 

 a piece of wood of the same size. In many cases it is 

 necessary to express numerically the ratio of the 

 weights of bodies ; this is done by comparing the weight 

 of each body with the weight of an equal volume of 

 water. 



The weight of the unit volume (l co ) of any substance is 

 its ABSOLUTE specific gravity. 



The number which expresses how many times heavier a 

 body is than an equal volume of some particular sub- 

 stance chosen as a standard of comparison, is called 

 the EELATIVE specific gravity of that body. 



The standard substance with which the specific 

 gravities of solid and liquid bodies are usually com- 

 pared is WATER. The term 4 specific gravity ' will 

 hereafter be used for c relative specific gravity.' 



A piece of glass which weighs 48^ has a volume of 

 20 CC ; hence an equal volume of water weighs 20 gr , and 

 the specific gravity of glass is therefore 2*4, for 



20 x 2-4 = 48, or \ ' =2 '4. The specific gravity of 

 20 



a body is found by dividing its absolute weight by that of 

 an equal volume of water. The specific gravity of bodies 

 lighter than water is a proper fraction. If the weight 

 of a cube of cork, with edges each 2 cm in length, and 

 hence a volume of 2 x 2 x 2 = 8 CC , is found to be 



2**, its spec. grav. is 25, for 8 of water weigh 8 gr , 



2 

 and - = 0*25. The specific gravity is thus an abstract 



number ; but it will be seen that it also expresses the 

 absolute weight in grammes of a cubic centimetre of 

 the body, and the weight of a cubic decimetre of the 



