592 GLOSSARY. DENSITY. 



two substances. Thus a cubic inch of platinum, the heaviest 

 metal known, very nearly weighs as much as two cubic inches of 

 silver. It is therefore said that the density of platinum is nearly 

 twice the density of silver. But silver is, in round numbers, ten 

 times heavier than water that is to say, one cubic inch of silver 

 weighs as much as ten cubic inches of water ; and platinum is 

 more than twenty times heavier than water ; so that if the density of 

 water be taken as a standard, and said to be 1.000, or, in popular 

 language, a thousand, then the specific gravity of silver is expressed 

 by 10.000, and the specific gravity of platinum is expressed by 

 20.000, or more popularly thus, with the same relative quanti- 

 ties silver, 10,000 ; platinum, 20,000 ; water being taken at 

 1,000 for 1,000 : 10,000 : : 1. : 10.; and 1,000 : 20,000 : : 1. 20. 

 Silver is really 10.5 heavier than water, and platinum twenty-one 

 times heavier. 



When we can discover the weight of any bulk or measure of a 

 body relative to a given weight of water, for example, to a thou- 

 sand grains of water, then we know its specific gravity that is to 

 say, we know how many grains of such body occupy the same 

 space as a thousand grains of water ; and if water be called 1000, 

 then the weight of the equal bulk of that other body in grains 

 expresses its specific gravity. Thus, a vessel that contains exactly 

 a thousand grains of distilled water at 60 Fahrenheit, and while 

 the barometer stands at 30, holds 793 grains of absolute alcohol, 

 whence it follows that the density of absolute alcohol is to the 

 density of distilled water under these circumstances as 793 : 1000 ; 

 or, in the language adopted in science, the specific gravity of abso- 

 lute alcohol is, if water be called 1000, 793, or .793, if water be 

 called 1.000, 



If the body, the specific gravity of which is required, be a solid 

 insoluble body, it is to be weighed first in air, and then in water. 

 Its weight in water is less than its weight in air by the weight of 

 its own bulk of water which it displaces. Thus we have obtained 

 a number denoting the weight of a bulk of water equal to the 

 bulk of the body under examination. We -have discovered, there- 

 fore, the comparative weights of equal bulks of water and the body 

 in question ; for the weight of that body in air, and the difference 

 between its weight in air and its weight in water, denote the com- 

 parative weights of equal bulks of that body and of water. 



If the body is lighter than water, it must be sunk by means 

 of a weight, and the calculation is rendered a little more 

 complex. 



If the body be soluble in water, some fluid, such as alcohol, 

 ether, or oil of turpentine, must be chosen, in which it is insoluble. 

 When the comparative weights of equal bulks of the body and 

 water have been obtained in any of these cases, the rule of three 



