IMIYSH \1. PROPERTIES 



259 



Hardness is an uncertain term, Imt in mineralogy it may be 

 taken to indicate the ra-e or difficulty \vitli which a mineral may 

 latched. It is a directional quality and not only varies with 

 the crystal form, but with the direction on the same crystal face. 

 The hardness of cyanite when tested on the macropinacoid parallel 

 to the vertical axis is 4-5, but tested on the same lace at right 

 angles to this direction it is nearly 7. When the hardness is tested 

 in all directions on a face like that of cyanite, and the value- 

 plotted in a curve, this 

 curve will bo found to 

 conform to and reflect 

 the symmetry of the face, 

 Fig. 373. 



Where it is wished to 

 determine the hardness 

 with some degree of ac- 

 curacy, the sclerometer 

 is used, the principle 

 of which depends upon 

 weighting a sharp point 

 of a very hard substance, 

 as a pointed diamond, 

 until it just scratches the 

 surface of the specimen when drawn across in any particular direc- 

 tion in which the hardness is wished to be determined. The weight 

 of the load which just produces a scratch is taken as a measure of 

 the hardness. Again, the material removed by the point under a 

 constant load after being drawn across the face a determined num- 

 ber of times may also be taken as a measure of the hardness, but 

 here the hardness will be inversely as the weight of material re- 

 moved. The loss of weight or the material removed is determined 

 by weighing the crystal. 



Hardness as a test in the identification of mineral species is deter- 

 mined by reference to a series of minerals arranged in a table, in 

 ascending scale, from 1, the softest, to 10, the hardest. The series 

 of ten test minerals is known as Moh's scale of hardness, a purely 

 arbitrary scale and in no way representing the true relation, as the 

 difference between 9 and 10 in the scale is ever so much greater 

 than the difference between 1 and 2. Moh's scale of hardness 

 below is also expressed in terms of hardness as determined by the 

 sclerometer, in which the hardness of the sapphire is taken as 1000. 



FIG. 373. Curve of Hardness on the Cube 

 Face of Fluorite. 



