Kip — Determination of the Hardness of Minerals. 25 



noting the weight required to force a point a given depth into 

 a surface to the laborious method of Anerbach, who regards 

 hardness as " the limiting elastic resistance (tenacity) of a body, 

 in case of contact of one of its plane surfaces with the spherical 

 surface of another body,'" and who would obtain a value for 

 hardness by multiplying the least value of the (central) pressure 

 per unit of area necessary to produce permanent set or rupture 

 at the center of the impressed surface by the cube root of the 

 radius of the sphere. 



3. In taking up the problem once more I have a three- 

 fold object in view. 1. To invite general acceptance of a single 

 definition of hardness, based upon the actual constitution of 

 minerals rather than upon abstract physical conceptions, which 

 will serve as a working hypothesis in determining its value. 

 2. To establish theoretically in conformity with the definition 

 the best method of investigation. 3. To put this method in 

 practice by means of suitable apparatus and adequate math- 

 ematical calculation. 



It is self-evident that we cannot expect uniformity of result 

 until we secure uniformity of aim, i. e. an agreement as to 

 what hardness is. By this is not meant an explanation of the 

 factors that combine to produce the quality (that is a problem 

 for pure physics), but merely an agreement as to what force or 

 forces must be used to overcome hardness. This, it will be 

 observed, is a simpler task than the measurement of the forces 

 upon which hardness depends. With these we cannot as yet 

 deal directly, or more properly singly. Fortunately there is 

 already a high degree of unanimity along mineralogists as to 

 what hardness means for them. This conception has been 

 crystallized in the brief but admirable definition given by Dana : 

 " Hardness is the resistance offered by a smooth surface to 

 abrasion," and with a slight improvement by the Century 

 Dictionary, as " the quality of bodies which enables them to 

 resist abrasion of their surfaces." To this conception of the 

 quality, which is not an off-hand generalization but a well-con- 

 sidered and well-tested definition I hold, not loosely and vaguely 

 but with all strictness. Other definitions are conceivable and 

 others have been given, but none defines so accurately what 

 the mineralogist understands by hardness and none adapts itself 

 better to the physical constitution of minerals as we conceive 

 it to be. To a person not familiar with the history of scler- 

 ometry this insistence upon a single clear-cut definition, in view 

 of the fact that others are possible, may seem superfluous if not 

 pedantic. Yet the lack of one or the failure to hold firmly to 

 one accepted in theory is accountable for much of the confusion 

 of conception and diversity of aim on the part of those who 

 have believed themselves to be working on the same problem. 



