OF CENTRAL CANADA PART I. 15 



6. FELDSPAR. 

 ,7. ROCK CRYSTAL. 



8. TOPAZ. 



9. CORUNDUM. 

 10. THE DIAMOND. 



In order to ascertain the hardness of a mineral by means of this 

 scale, we attempt to scratch the substance, under examination, by the 

 different specimens belonging to the scale, beginning with the hardest, 

 in order not to expose the specimens to unnecessary wear. Or, 

 proceeding in another manner, we take a fine file, and compare the 

 hardness of the mineral with that of the individual members of the 

 scale, by drawing the file quickly across them. The comparative 

 hardness is estimated by the resistance offered to the file ; by the 

 noise occasioned by the file in passing across the specimens ; and by 

 the amount of powder so produced. The degree of hardness of the 

 mineral is then said to be equal to that of the member of the scale 

 with which it agrees the nearest. Thus, if the mineral agree in 

 hardness with fluor-spar, we say, in its description, H (or hardness) 

 = 4. If, on the other hand, it be somewhat softer than fluor-spar, 

 but harder than calcareous spar, we say, H = 3.5. Finally, if, as 

 frequently happens, the hardness of a mineral vary slightly in 

 different specimens, the limits of the hardness are always stated. 

 Thus, if in some specimens, a mineral agree in hardness with calc- 

 spar. and in others with fluor-spar, we say, H = 3 to 4 ; or, more 

 commonly, H = 3 4. If the hardness be very rigorously tested, 

 it will frequently be found to differ slightly on different faces of a 

 crystallized specimen, or on the broad faces and the edges of the laminse 

 of foliated specimens ; but this, so far as regards the simple deter- 

 mination of minerals, is practically of little moment. 



As the minerals of which the scale of Mohs consists are not in all 

 places obtainable, or always at hand when required, the author of this 

 work devised, many years ago, a scale of hardness so contrived as to 

 agree closely enough for practical purposes with that of Mohs, whilst 

 exacting for its application only such objects as are always to be 

 met with. The following is tho scale in question : its use explains 

 itself. 



