96 FIELD GEOLOGY. 



yield much powder, and but little noise. The noise 

 and amount of powder should be compared with that 

 produced by minerals which are used as standard 

 examples. The trial may also be made by endeavour- 

 ing to scratch the specimens enumerated in the list 

 with the mineral under examination. If, for example, 

 the mineral will scratch Felspar and will not scratch 

 Quartz, it will have a hardness between 6 and 7. 



" During these trials the ' colour ' and ' lustre ' of the 

 streak should also be noticed."* " "When a mineral is 

 scratched, the colour of the scratched surface frequently 

 differs from that of the original surface, and if the 

 abraded powder be rubbed on paper, it leaves a mark 

 of # peculiar colour. This is called the streak, and in 

 some of the metallic minerals it is very characteristic."^ 

 Note. In all the Tables here given only an ap- 

 proximation is intended, the averages of hardness 

 and specific gravity inserted and the more simple 

 of the tests for solubility and blow-pipe analysis. 

 For more accurate and for ultimate determination 

 of any particular specimen recourse should be had 

 to some of the works specially devoted to the 

 , subject or to the professed mineralogist. A list is 

 given at page 11*7 of some of the works which may 

 with advantage be consulted. 



Specific Gravity. The " specific gravity " of a body 

 being the proportion that its weight bears to that of 

 an equal bulk of distilled water (the latter being taken 

 as unity), it is evident that if such body be weighed, 

 first in air, then in distilled water, the resulting loss 



* " Mineralogy." Kutley. Murby's Series, 1874, p. 39. 

 t " Mineralogical Tables." Jewesbury. Murby's Series, 1873, 

 p. 9. 



