16 MINERALS AND GEOLOGY 



Chapman's Convenieat Scale of Hardness, to correspond with 



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 a : 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. qr. = -? 



a w 



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

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



- "-'..- 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. 



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