158 



minerals. The thin edge of a splinter sometimes 

 transmits light in a slight degree ; in which case 

 it is called translucent. 



ACTION OF ACIDS. 



The extrication of carbonic acid from rocks, 

 by the action of dilute nitrous or muriatic acid, 

 and the greater or less facility with which it is 

 extricated, sometimes offer convenient methods 

 of discriminating certain varieties of limestone. 



SPECIFIC GRAVITY. 



The specific gravity of rocks must be ascer- 

 tained in the usual manner, and ought to be 

 stated in describing a rock, where the weight is 

 to be mentioned ; as vague comparative terms 

 convey no ideas on this subject. This character 

 is, however, of little use in the discrimination of 

 rocks. 



COLOUR. 



In concluding this catalogue of the obvious 



