14 MEMOIR ON EMERY. 



structure, not unlike certain varieties of very compact blue 

 limestone. x 



Fracture. — The fracture of emery is tolerably regular, and 

 the surface exposed is granular, of an adamantine aspect ; it is 

 exceedingly difficult to break when not traversed by fissures or 

 not of a lamellated structure, as much of that from Nicaria. 

 When reduced to powder it varies in color from that of a dark- 

 gray to black. The color of its powder affords no indication 

 of its commercial value. 



The powder examined under the microscope shows the distinct 

 existence of the two minerals, corundum and oxide of iron, 

 which appear inseparable, as the smallest fragment contains 

 the two together. 



Magnetism.' — As it is natural to suppose, all specimens of 

 emery affect more or less the magnetic needle ; in some the 

 magnetism is barely perceptible, in others it amounts to strong- 

 polarity. 



Odor. — Emery when moistened always affords a very strong 

 argillaceous odor — even the most compact varieties. 



Specific gravity. — The different varieties do not vary much 

 in their specific gravity, it being always in the neighborhood 

 of 4. The specific gravity of various specimens will be given 

 on a following page. 



Hardness. — The hardness of emery is its most important 

 property, as to it is due the value of this substance in the arts. 

 For this reason I have devoted much time and attention to the 

 determination of it. In a mineralogical sense, its hardness is 

 not difficult to determine ; for if we try different varieties of 

 emery by scratching agate or other hard substance, the effect 

 will naturally be very nearly the same, for in every case it will 

 be some point of corundum that has produced the scratch. If, 

 however, we happen not to rub a point of corundum against 

 the agate, no effect will be produced on the latter, but the 

 emery will yield. As this method leads to no practical result, 

 I have sought out another, which may properly be called one 

 for determining the effective hardness of emery and corundum, 

 and is as follows. 



Fragments are broken from the piece to be examined, and 

 crushed in a diamond mortar with two or three blows of a 

 hammer, then thrown into a sieve (the one employed had four 



