116 TERMINOLOGY. 



. 98. HARDNESS. 



Hardness is the resistance of solid minerals to the dis- 

 placement of their particles, the magnitude of which con- 

 stitutes their degree of Hardness. 



The character now under consideration is one of the most impor- 

 tant possessed by minerals, for the purposes of their determination. 

 We have not, hitherto, been able to ascertain the positive hardness 

 of minerals, from the difficulty of establishing an accurate scale for 

 the degrees of hardness. All the means we at present possess, for 

 arriving at this end, consist merely in the general comparison of the 

 known with the unknown. 



The existence of differences in the degrees of hardness among 

 minerals, is very easily ascertained, by the simple experiment of 

 scratching one of them by the other. Thus, a sharp angle of Quartz 

 will produce a deep furrow in Calcareous Spar; whilst a sharp corner 

 of the latter species does not injure the surface of the former. ^ Ac- 

 cordingly, we conclude that Quartz is harder than Calcareous Spar; 

 and, in general, that of two minerals, the harder one scratches the 

 other, but cannot, inversely, be scratched by it. 



By proceeding upon this principle, a scale for the degrees of hard- 

 ness has been made out, which possesses sufficient accuracy for the 

 purposes of mineralogy. This is effected by choosing a certain num* 

 ber of suitable minerals, and arranging them in such an order that 

 every preceding one is scratched by that which follows it, while the 

 latter does not scratch the former. 



The scale is as follows : 



1. Talc, The common green or greenish white varieties, 



2. Gypsum. An uncrystallized variety. This degree of hard* 

 ness is exactly that of Rock Salt, which mineral, therefore, may be 

 substituted for Gypsum, in the determination of hardness, or it may 

 assist in the selection of the proper variety of Gypsum to be employ- 

 ed as a term of comparison. 



3. Calcareous Spar, or a cleavable variety of Carbonate of Lime, 

 Bitter Spar cannot be employed as a substitute, its hardness being 

 somewhat greater than Calcareous Spar. 



4. Fluor. Any cleavable variety. 



5. Apatite. Crystals possessing a conchoidal fracture? 



6. Feldspar. A cleavable variety of Adularia. 



7. Quartz. Limpid and transparent, 



