SPECIFIC GRA VITY. 135 



ECTION HI, 



SPECIFIC GRAVITY. 



241. What is Specific Gravity ? ^e specific 

 gravity of a body is the ratio between its weight 

 and the weight of a like volume of some other 

 substance taken as a standard. 



212. Standard of Specific Gravity. The 



standard taken must be invariable. For solids and liquids, 

 the standard adopted is distilled water at a tem- 

 perature of 4 C., or 39.2 F. For aeriform bodies, the 

 standard is air or hydrogen. 



(a.) The water is to be distilled, or freed from all foreign sub- 

 stances, because the weight of a given quantity of water varies with 

 the substances held in solution. It is to be at a fixed temperature 

 because of the expansion by heat. The temperature above men- 

 tioned is that of water at its greatest density. In cases where air or 

 hydrogen is taken as a standard, the additional condition of atmos- 

 pheric pressure must, for obvious reasons, be recognized. The pres- 

 sure to which all observations in this country are reduced is that 

 recorded by 30 inches (760 mm.) of the barometer. 



243. Elements of the Problem. For solids 

 or liquids, the dividend is the weight of the given 

 body ; the divisor is the weight of the same bulk 

 of water ; the quotient, which is an abstract number, is 

 the specific gravity, and signifies that the given body is so 

 many times heavier than the standard. The weight of the 

 same bulk of water is found sometimes in one way and 

 sometimes in another, but in every case it is the divisor. 

 By grasping and keeping this idea, you will avoid much 

 possible confusion. Of course, when any two of these 

 three are given, the third can be found. 



