. 208. PHYSICAL PROPERTIES OF MINERALS. 305 



First we try, with a corner of the given mineral, to 

 scratch the members of the scale, beginning from above, 

 in order that we may not waste unnecessarily the spe- 

 cimens representing lower members. After having thus 

 arrived at the first, which is distinctly scratched by the 

 given mineral, we have recourse to the file, and compare 

 upon it the hardness of this degree, that of the next higher 

 degree, and of the given mineral. Care must be taken 

 to employ specimens of each of them nearly agreeing in 

 form and size, and also as much as possible in the quality 

 of their angles. From the resistance these bodies oppose 

 to the file, and from the noise occasioned by their passing 

 over it, we argue with perfect security upon their mutual 

 relations in respect to hardness. The experiment is repeat- 

 ed with all the alterations thought necessary, till we may 

 consider ourselves arrived at a fair estimate, which is at 

 last expressed by the number of that degree with which it 

 has been found to agree nearest, the decimals being like- 

 wise added, if required. 



The files answering best for the purpose are fine and 

 very hard ones. Their absolute hardness is of no conse- 

 quence ; hence every file will be applicable, whose hard- 

 ness 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 minerals, but the hardness of ano- 

 ther mineral, by the medium of the file. From this ob- 

 servation 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, scratch- 

 ing with the nail, &c. 



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 general every 

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