362 J. Lawrence Smith on Emery. 
 - Magnetism.—As‘itis natural to suppose all: specimens of em- | 
ery affect more or less the magnetic needle; in some the magnet- | 
ism 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.—T he different varieties do. not vary much in 
__ their specific gravity, it béing always in the neighborhood of 4. 
The specific gravity of various specimens will-be given-on a 
following page. . " a Poe 
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 deter- 
mination of it. ‘Ina mineralogical sense its hardxess is not diffi- 
enlt 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 hap- 
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sieve; the operation is repeated until all the emery has passed 
through the sieve. ‘The object of giving but two or three blows 
at a time is to avoid crushing any of the emery to too fine a 
powder. Ss 
Thus pulverized it is intimately mixed and acertain portion of 
it is weighed, (as I operated with a balance ‘sensible’to a milli- 
gramme, the quantity used never exceeded a gramme.) ‘To test 
the effective hardness of this, a circular piece of glass about four 
inches in diameter and a small agate mortar are used. The glass 
is first weighed and placed ona piece of glazed paper; the pul- 
verized emery is then thrown on it little by little, at each time 
rubbing it against the glass with the bottom of the agate mortar. 
The emery is brushed off the glass from time to time with a 
feather, and when all the emery has been made to pass once over 
the glass, it is collected from the paper and made to pass through 
she same operation which is repeated three or four times. e 
glass is then weighed, after which it is subjected to the same Op- 
ration as before, the emery being by this time reduced to an im- 
palpable powder. This series of operations is continued unt 
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