PHYSICAL PROPERTIES OF MINERALS. 117 



8. Topaz. In crystals. 



9. Corundum. The easily cleavable varieties. 

 10. Diamond. 



The minerals representing the units of this scale, have been cho- 

 sen among those species which may be most readily obtained. It 

 will be perceived, however, that the intervals between the mem- 

 bers of the scale are not every where of the same magnitude. 

 Diamond is evidently much harder, if compared with Corundum, 

 than Fluor with Calcareous Spar. This however leads to no incon- 

 venience, in the case above mentioned ; for there exists no mineral 

 of a hardness intermediate between the degrees represented by the 

 two first of these species. The interval between Fluor and Feld- 

 spar is also greater than it should be ; and, in this case, it would be 

 desirable to have another mineral to substitute for Fluor, whose 

 hardness should be such as to divide more equally the interval be- 

 tween Calcareous Spar and Feldspar. Still, wilh these imperfec- 

 tions, the scale is used to the highest advantage. The degrees of 

 hardness are expressed by means of those numbers, which, in the 

 above enumeration, are prefixed to them. Thus, the hardness of 

 Feldspar is =6, that of Corundum =9. 



The intervals between each two subsequent members may be 

 divided into ten equal parts ; and these tenths determined by es- 

 timate. It will very seldom be required to value the hardness to 

 more or less than 0.5; but it will always be possible to proceed so 

 far as we find It necessary to answer our purpose. 



The state of liquidity may be considered as the zero of the scale. 



If, in employing the scale, we endeavor to find the degree of 

 hardness of a given mineral, by trying which member of the series 

 is scratched by it, and which of them injures the surface of the 

 given one, it will appear that the specimens employed, shouH pos- 

 sess certain properties, in many cases difficult to be found. They 

 should all have faces perfectly smooth and even, and solid angles or 

 corners of the same form, and be equally hard. 



As to the faces, those produced by cleavage seem the most eligible, 

 if they possess a pretty high degree of perfection. Faces of crys- 

 tallization are commonly uneven or streaked ; cut and polished fa- 

 ces, however, in many instances, shew a less degree of hardness 

 than the mineral really possesses. 



It is still more difficult to obtain the corners with the constant 

 quality which is requisite. Even in a determined form, these are 

 sometimes liable to be so much influenced by structure, that they 

 give very uncertain results. In this respect, the solid angles of the 



