306 TERMINOLOGY. . 208. 



circumstance which might influence the hardness, must be 

 duly attended to, if we intend to arrive at a useful and 

 correct result. 



Minerals that cleave with particular facility in only one 

 direction, very often shew a less degree of hardness upon 

 the perfect face of cleavage, than in other directions. Pris- 

 matic Disthene-spar sometimes is scratched by octahedral 

 Fluor-haloide upon the eminent face of cleavage, whilst an 

 angle of the very same individual scratches not only rhom- 

 bohedral Fluor-haloide, but even sometimes prismatic 

 Feld-spar. If we intend to determine a mineral of this de- 

 scription by the help of the Characteristic, it will be the 

 best plan to take a mean term between the two degrees 

 measured, or rather to keep nearer to the higher one. It 

 would be wrong to receive them into a scale of hardness, 

 like the preceding one, since this would betray a want 

 of acquaintance with the scale itself, and with its employ, 

 ment. 



Supposing all the precautions necessary in determining 

 the degrees of hardness to have been taken, and the cir- 

 cumstances well attended to, which might have exercised 

 some influence ; we find that those individuals which be- 

 long 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. This 

 observation seems to be contradicted by the authority of 

 several mineralogical works. But there are indeed few 

 subjects with regard to the properties of minerals, which 

 have been treated with more indifference or even careless- 

 ness than their hardness, and on this account little or no 

 credit is due to what most of the mineralogical works con- 

 tain of its indications. 



KIRWAN, DE LA METHERIE, and HOME DE L'IsLE, 

 have each endeavoured to construct scales of hardness. A 

 comparative table of the hardness of different substances. 



