PHYSICAL PROPERTIES OF MINERALS. 119 



resistance these bodies oppose to the file, and from the noise occa- 

 sioned by their passing over it, we infer with perfect security, their 

 mutual relations in respect to hardness. The experiment is repeat- 

 ed with all the alterations thought necessary, till we may consider 

 ourselves as having arrived at a fair estimate, which is at last expres- 

 sed by the number of that degree with which it has been found to 

 agree nearest, the decimals being likewise added, if required. 



The files answering best for the purpose are fine and very hard 

 ones. Their absolute hardness is of no consequence ; hence, every 

 file will be applicable, whose hardness is in the necessary relation 

 with that of the mineral. For it is not the hardness of the file with 

 which we have to compare that of the mineral, but the hardness 

 of another mineral by the medium of the file. From this observa- 

 tion it appears, that the application of the file widely differs from 

 the methods of determining the hardness of minerals which have 

 hitherto been in use ; as scratching glass, striking fire with steel, 

 cutting with a knife, scratching with the nail, Sac. 



Besides an appropriate form, there is another necessary property 

 of the minerals to be determined, consisting in their state of purity. 

 Neither the degree of hardness, nor that of specific gravity, can 

 be correctly ascertained, if we employ impure substances. For the 

 same reason, it would be wrong to make use of minerals which 

 have undergone a total or even partial decomposition ; and in gen- 

 eral, every circumstance which might influence the hardness must 

 be duly attended to, if we hope to arrive at a useful and correct 

 result. 



Minerals that cleave with more facility in one direction than in 

 any other, often shew a less degree of hardness upon the perfect 

 face of cleavage than in other directions. This is exemplified in 

 Cyanite and Mica. If we are engaged in the determination of a 

 mineral by the help of the characteristic, it will be necessary to take 

 a mean term between the two degrees measured, or rather, to lean 

 towards the higher one. 



Supposing all the precautions necessary in determining the de- 

 grees of hardness to have been taken, and the circumstances well 

 attended to, which might have exercised some influence ; we find 

 that those individuals which belong to one and the same species, 

 admirably agree with each other in respect to this property ; and 

 that deviations from an exact coincidence, if they happen to occur, 

 do not take place, per saltum, but that they are joined with each 

 other by intermediate members. These members produce a series, 

 in most cases between very narrow limits. 



