Kip — Determination of the Hardness of Minerals. 31 



Static pressure tests, producing as they do penetration or 

 fracture but not abrasion, are excluded from the outset. 



The rotary method under which are included the Rotations- 

 sklerometer of M tiller and the rotating discs of Jannettaz and 

 Goldberg as well as the Mesosklerometer of Pfaff and Jaggar's 

 Microsklerometer, is likewise inadmissible, certainly in the form 

 in which it has been employed hitherto. For when pressure 

 alone is measured it becomes at once merely a modified form 

 of static pressure test. And the practical difficulties that 

 would confront us if we were to attempt to measure the force 

 of rotation, involving friction at so many points, to calculate 

 the increment of resistance as depth increased and to maintain 

 molecular dislocation as opposed to mass dislocation, on the one 

 hand and polishing of the surface on the other, would be exceed- 

 ingly great if not insurmountable. Grinding a surface with a 

 standard sand is open to the objection that no means is offered of 

 guarding against mass dislocation, the sand becomes at once 

 adulterated with particules of the abraded substance, there is 

 no certainly that the sand itself has the same force of attack in 

 any two tests and there is no means of determining definitely 

 when the sand is " dead." 



We are led by a process of elimination to the abrading 

 method par excellence, at once the simplest and most delicate, 

 which for want of a better name is known as the scratch 

 method. This fulfills, or can be made to fulfill, all the require- 

 ments enumerated at the head of this paragraph, and permits, 

 furthermore, of distinction between molecular and mass dis- 

 placement, in so far as we can deal at all with submicroscopic 

 divisions of matter. In reviewing" the devices hitherto em- 

 ployed we find that none satisfies entirely the requirements that 

 may justly be made of such an instrument. Several fail to 

 provide any means of measuring pull as well as pressure, none 

 of them has been actually used so as to measure both forces 

 at the same time, and those that might have been so used afford 

 no means of distinguishing between the forces actually produc- 

 ing abrasion and those expended in other ways or for other 

 purposes. 



10. To meet the demands imposed by our definition of 

 hardness and by our conception of the physical structure of 

 minerals the apparatus described below has been designed. 



The diamond point resting on the surface of the mineral is 

 balanced by a weight hanging directly beneath it and suspended 

 from four arms running out from the short brass cylinder into 

 which the diamond point and its holder are screwed. Pres- 

 sure on the point is regulated by the amount of the weight w 

 (fig. 3). The mineral m is drawn in the direction ab by means 

 of a thread passing over a pulley p and ending in a weight x. 

 A thread attached to one of the above mentioned arms at just 



