MEMOIR ON EMERY. 15 



hundred holes to the square centimetre) ; the portion passing 

 through is collected, and that remaining on the sieve is again 

 placed in the mortar and two or three blows given, then thrown 

 into the 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. 



Thus pulverized, it is intimately mixed and a certain portion 

 of it is weighed (as I operated with a balance sensible to a 

 milligramme, 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 on a piece of 

 glazed paper ; the pulverized 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 the same operation, which is repeated three or four 

 times. The glass is then weighed, after which it is subjected 

 to the same operation as before, the emery being by this time 

 reduced to an impalpable powder. This series of operations is 

 continued until by repeated weighing the loss sustained by the 

 glass is reduced to a few milligrammes. The total loss in the 

 glass is then noted, and when all the specimens of emery are 

 submitted to this operation under the same circumstances we 

 get an exact idea of their relative hardness. 



The blue sapphire of Ceylon was pulverized and experi- 

 mented with in this way; it furnished me with a unit of 

 comparison by which to compare the results obtained. This 

 0}3eration is long but certain, and for the harder varieties of 

 emery it is necessary to repeat the rubbing six or seven times, 

 and it requires nearly two hours for completion. 



The results that I have obtained are interesting, and have 

 furnished me with the means of forming conclusions that I 

 could not otherwise have come at. 



Glass and agate have not been chosen for this experiment 

 without a certain object, as experiments were first made with 

 two pieces of agate, with two pieces of glass, and with metal 



