16 MEMOIR ON EMERY. 



and glass. The agates were found too hard, as they crushed 

 the emery without producing hardly any abrasive effect; the 

 others were found not to crush the emery sufficiently, making 

 the experiment tedious and long. With the glass and agate we 

 have a hard substance which crushes the emery, and in a cer- 

 tain space of time reduces it to such an impalpable state that 

 it has no longer any sensible effect on the glass, and on the 

 other hand the glass is soft enough to lose during this time 

 sufficient of its substance to allow of accurate comparative 

 results. In the employment of this method in the arts it 

 would not be necessary to go to the sapphire for a standard of 

 comparison; any good emery would answer the purpose quite 

 as well. 



It must be understood that this method' of coming at the 

 abrasive effects of emery does not furnish the mineralogical 

 hardness of this substance, by which we understand the hard- 

 ness of any individual particle, as evinced by its effect on a 

 substance of less hardness, without regard to the molecular 

 structure of the mineral. Two minerals possessing the same 

 hardness but differing in structure, one being friable and the 

 other resisting, will be found very different in their abrasive 

 effects; for instance, break a piece of quartz in two, subject 

 one of the pieces to a white heat, and after cooling compare 

 the two by rubbing the point against some hard substance; 

 both will be found to scratch equally well. Then try the two 

 in a state of powder by rubbing them between two pieces of 

 glass that have been weighed, and the difference of their abra- 

 sive effects will be found very great, because the one subjected 

 to the fire is exceedingly friable and becomes readily crushed 

 to an impalpable powder. This fact is eminently true with 

 reference to emery, many specimens of which containing the 

 same amount of corundum differ somewhat in their effective 

 hardness, owing to the more or less compact structure of the 

 corundum. 



By the method with the agate and glass I have found the 

 best emery capable of wearing away about one half its weight 

 of the glass (that used was the common French window-glass). 

 The sapphire under the same circumstances wears away more 

 than four fifths of its weight. A tabular view of the results 

 will be given a little farther on. 



