18 



MINERALS AND GEOLOGY 



purchased anywhere for a quarter of a dollar ; or otherwise we appl 

 the specimen to a properly suspended magnetic needle. In this 

 manner many of the black granular masses which occur so frequently 

 in our Gneissoid or Laurentian rocks, and in the boulders derived 

 from these, may easily be recognised as magnetic iron ore.* Most 

 specimens of this mineral (and also of magnetic pyrites) exhibit 

 "polarity," or attract, from a given point, one end of the needle, and 

 repel the other. 



Taste. This is a very characteristic although limited property, 

 being, of course, exhibited only by soluble minerals. In these, the 

 taste may be saline, as in Rock Salt ; or bitter, as in Epsom Salt ; 

 or metallic, as in Sulphate of Iron, and so forth. 



B. CHEMICAL CHARACTERS. 



The chemical characters of principal use in the determination of 

 minerals comprise the phenomena developed by the action of acids, 

 and those produced by the application of the blowpipe. Before 

 referring to these characters, the reader should be familiar with cer- 

 tain chemical terms of common employment in Mineralogy. 



A substance of any kind, whether of natural or artificial formation, 

 must be either a simple or a compound substance. If the former, it 

 cannot be decomposed or subdivided into more simple bodies by any 

 process of art. If compound, on the other hand, a decomposition of 

 this kind may be more or less readily effected. Thus, whilst from a 

 piece of sulphur, copper, or iron, if pure, nothing but sulphur, copper 

 or iron respectively, can be extracted, a piece of copper pyrites will 

 yield all three of these substances each, as before, resisting further 

 subdivision. Hence sulphur, copper, and iron are regarded as simple 

 substances, whilst copper pyrites is a compound body. These so-called 

 "simple" substances, it must be understood, may not be, and probably 

 are not, absolutely simple; but they are simple, id est, undecomposable, 

 in the present state of science. They are often known as Elements. 

 Up to the present time between sixty and seventy have been 

 recognized, but many occur only in a few rare minerals. Some 

 oxygen, nitrogen, chlorine, fluorine, hydrogen exist in the free 

 state as gases ; two at ordinary temperatures are liquid ; the rest 



* The other dark -coloured cleavable masses, in these rocks, consist mostly of mica, ho n- 

 blende, or tourmaline. 



