16 MINERALS AND GEOLOGY 



Cha})man's Convenieat Scale of Hardness, to correspond with that 



of Mohs. ' 



1. Yields easily to the nail. 



2. Does not yield to the nail. Does not scratch a copper coin.* 



3. Scratches a copper coin, but is also scratched by one, being of 



about the same degree of hardness. 



4. Not scratched by a copper coin. Does not scratch glass (ordinary 



window-glass). 



5. Scratches glass very feebly. Yields easily to the knife. 



6. Scratches glass easily 



7. Yields with difficulty to the edge of a file. 



8. 9, 10. Harder than flint or rock-crystal. 



Convenient objects for the estimation of degrees of hardness above 

 No. 7 cannot be easily obtained ; but that is of little consequence, as 

 there are but few minerals 'which exhibit a higher degree, and these 

 are readily distinguished by other characters. 



Specific Gravity. This is also a character of great value in the 

 determination of minerals. The specific gravity of a body is its 

 weight compared with the weight of an equal bulk of pure water. 

 In order to ascertain the specific gravity of a mineral, we weigh the 

 specimen first in air, and then in water. The loss of weight in the 

 latter case exactly equals the weight of the displaced water, or, in 

 other words, of a volume of water equal to the volume of the mineral. 

 The specific gravity of pure water, at a temperature of about 62, 

 being assumed to equal 1, or unity, it follows that the specific gravity 

 of a mineral is obtained by dividing the weight of the latter in air 

 by its loss of weight in water. Thus, if a = the weight in air, and 

 w = the weight in water, G, or sp. gr. = 



a w 



JSxample. A piece of calcareous spar weighs 66 grs. in air, and 

 42 grs. when immersed in rain or distilled water. Hence its sp. gr. 

 . _fL- = ? = 2-75. 



66 42 24 



The weight of the mineral may be ascertained most conveniently, 

 and with sufficient exactness for general purposes, by a pair of small 

 scales such as are commonly called " apothecaries' scales." These may 



* Thas is, an old-fashioned copper coin, not a modern bronze coin. The scale was published 

 in 1843. 



