118 TERMINOLOGY. 



Tetrahedron, and those of the octahedron of Fluor, shew quite dif- 

 ferent results. The corners of compound varieties, in which the 

 individuals become impalpable or disappear, such as Chalcedony, 

 Flint, and others, are commonly found very powerful, much more 

 so than the similarly formed corners of simple varieties. But if 

 the composition is still observable, the particles very often separate 

 in the experiment of scratching another mineral, and the corner of 

 a compound mineral cannot produce the effect of that of the simple 

 mine al. The application of the edges is subject to similar difficulties. 



But the experiment of merely scratching one substance by an- 

 other, has been found not to lead to the most accurate determination 

 of the hardness of minerals. This is powerfully assisted by having 

 recourse to a file, in the manner presently to be described. 



If we take several specimens of one and the same mineral, and 

 pass them over a fine file, we shall find, that an equal force will 

 every where produce an equal effect, provided, that the parts of the 

 mineral in contact with the file be of similar size, so that the one 

 does not present to the file a very sharp corner, while the other is 

 applied to it by a broad face. It is necessary also, that the force 

 applied in this experiment, be always the least possible. Every per- 

 son, however little accustomed, will experience a very marked differ- 

 ence, if comparatively trying in this w r ay any two subsequent mem- 

 bers of the above scale, and thus, the difference in their hardness 

 will be easily perceived. A little practice is sufficient for render- 

 ing these perceptions more delicate and perfect, so that in a short 

 time, it is possible to determine differences" in the hardness very 

 much less those between two subsequent members of the scale. 



Upon this is founded the application of the scale ; the general 

 principle of which consists in this, that the degree of hardness of 

 the given mineral is compared with the degrees of hardness of the 

 members of the scale, not immediately, by their mutual scratching, 

 but mediately through the file, and determined accordingly. 



The process of this determination is as follows: 



First we try, wilh 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 specimens representing lower mem- 

 bers. 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 size and form, and 

 also as much as possible in the quality of their angles. From the 



