304 TERMINOLOGY. . 208. 



such as Calcedony, Flint, and others, are commonly found 

 very powerful, much more so than similarly formed corners 

 of simple varieties. But if the composition is still observ- 

 able, 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 

 mineral. The application of the edges is subject to similar 

 difficulties. 



Numerous experiments of determining the degree of 

 hardness, by the mere scratching of one substance with 

 the other, have completely established, that this process 

 alone is not sufficient, if we intend to make a more sure 

 and extensive application of the characters that may be 

 taken from hardness, than that which has hitherto been 

 common in Mineralogy. 



But 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 everywhere produce an equal effect, pro- 

 vided that the parts of the mineral in contact with the file 

 be of a 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 person, however little accustomed, will experience 

 a very marked difference, if comparatively trying in this 

 way any two subsequent members of the above scale, and 

 thus the difference in their hardness will be easily perceiv- 

 ed. A short practice is sufficient for rendering these per- 

 ceptions more delicate and perfect, so that in a short time 

 it is possible to determine differences in the hardness very 

 much less than those between two subsequent members of 

 the scale. 



Upon these observations 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 com- 

 pared 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 : 



