162 NATURE-STUDY LESSONS. 



quartz as compared with that of an equal volume of 



water. 



The weight of a substance as compared with an equal volume of water 

 is its specific gravity. 



20. In a similar way determine the specific gravity of 

 iron pyrites, and, if there is gold enough for the purpose, 

 also of gold. 



21. {a) Compare the specific gravities of gold, iron 



pyrites and quartz. 

 (b) Infer from this why gold is found in the layer 

 of sand resting on the rock in the beds of 

 creeks in some gold-mining regions. 



XLVI.-MINERALS. 



Observations on minerals may be begun by compari- 

 son of the qualities of two or more common species ; 

 for example, water and clay, clay and chalk, mica and 

 anthracite, mica and limestone, limestone and copper, 

 limestone and quartz. 



A cabinet of standard specimens for comparative 

 determinations of lustre and hardness is desirable, and, 

 although not indispensable for the purposes of nature- 

 study, it is necessary for even elementary study of 

 mineralogy. 



The extent to which a study of minerals may be 

 carried in a school will depend on its equipment for 

 heating, crushing, weighing and otherwise manipulating 

 the specimens. 



Having studied the minerals mentioned in Lessons 

 XLIII and XLIV, other minerals should be examined 

 and as many of the following points noted as possible :— 



