.808. PHYSICAL PROPERTIES OF MINERALS. 803 



ent degrees of hardness, and on that account also it is 

 very difficult to be obtained. Yet all these imperfections 

 are by no means prejudicial to the useful employment of 

 the scale. 



The degrees of hardness are expressed by means of those 

 numbers which in the above enumeration are prefixed to 

 them. Thus the hardness of rhombohedral Lime-haloide is 

 = 3, that of rhombohedral Corundum = 9. 



The intervals between each two subsequent members may 

 be divided into ten equal parts ; and these tenths determin- 

 ed by estimate. It will very seldom be required to value 

 the hardness to more or less than 0. 5 ; but it will al- 

 ways 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 endeavour to find the de- 

 gree 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 should possess certain proper- 

 ties, 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 durable. 



As to the faces, those produced by cleavage seem the 

 most eligible, if they possess a pretty high degree of per- 

 fection. Faces of crystallisation are commonly uneven or 

 streaked ; cut and polished faces, however, in many in- 

 stances 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 required. 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 tetrahedron, and those 

 of the octahedron of octahedral Fluor-haloide, shew quite 

 different phenomena. The corners of compound varieties, 

 in which the individuals become impalpable or disappear, 



