120 TERMINOLOGY. 



. 99. SPECIFIC GRAVITY. 



By the Specific Gravity of minerals, is understood the 

 relation which subsists among them, under equal volumes, 

 as respects their absolute weights. 



The determination of the specific gravity depends upon the com- 

 parison between absolute weights and volumes. They cannot be 

 instituted at all, or at least not with sufficient accuracy, except by 

 the aid of appropriate instruments. 



In ascertaining the specific gravity of minerals, water has been 

 agreed upon as the fixed standard of comparison. This preference 

 has been given from the remarkable facility with which we can 

 compare its weight with that of all other minerals, under equal 

 volumes, in consequence of the discovery of Archimedes, that when 

 a body is immersed in water, it loses a portion of its weight equal 

 to that of the volume of water it displaces, which volume is pre- 

 cisely equal to its own : accordingly, if a body, after having been 

 weighed in air, be weighed in water, the loss of weight which it 

 sustains will necessarily indicate that which belongs to a volume of 

 water exactly equal to its own. Therefore, if we represent the 

 specific gravity of water by any number whatever, we shall obtain 

 the specific gravity of the body weighed, by means of the following 

 proportion: the weight lost by the body weighed, (or, what is the 

 same thing, the weight of a volume of water equal to it,) is to the 

 absolute weight of the body, as the number chosen to represent the 

 specific gravity of water, is to the specific gravity of the body 

 weighed. The number which has been selected to represent the 

 specific gravity of water is 1. 



The specific gravity of water being liable to vary, from foreign 

 substances it often contains, and from an alteration of temperature, 

 it is obvious it cannot with propriety be made a standard of compari- 

 son, except when perfectly pure, and at a given temperature. For 

 these reasons, distilled water only is employed in ascertaining the 

 specific gravity of minerals, and at a temperature varying as little 

 as possible from 60 F. 



Several instruments are employed in taking the specific gravities 

 of minerals, some of which have reference to the state of the min- 

 eral, whether solid, fluid or gaseous. As it will very rarely be ne- 

 cessary to resort to the use of the present character in the deter- 

 mination of liquids and expansible fluids, the arrangements adopted 

 in ascertaining it, in these bodies, will not be described in this 



