MEMOIR ON EMERY. 17 



CHEMICAL COMPOSITION OF EMERY. 



This substance consisting of a mixture of corundum and 

 oxide of iron in various proportions, it is easy to see what its 

 composition must be. Yet the chemical examination of this 

 mineral, taken in connection with other properties, is not de- 

 void of interest. 



For the purpose of analysis the emery was reduced to a 

 state of powder, in the manner alluded to in speaking of its 

 hardness, with a diamond mortar and sieve. This powder was 

 dried for twenty-four hours over sulphuric acid ; a gramme 

 was then weighed in a small platinum crucible of about one 

 fourth of a cubic inch in capacity, fitted with a cover that 

 adapted itself well to it. This small crucible was placed in 

 another of earth, and the space between the two filled with 

 pulverized quartz which also covered the smaller one to the 

 depth of half an inch. Common sand was not used, because 

 during the heating some particles might adhere to the platinum 

 crucible by a semifusion ; nor was powdered charcoal employed, 

 because it protected the mineral no better than the pulverized 

 quartz from contact with the air, at the same time a little risk 

 was run in decomposing a small amount of the iron. 



Thus arranged, the crucibles were heated to a bright-red for 

 from thirty minutes to one hour. After cooling the platinum 

 crucible was carefully withdrawn and weighed. The loss fur- 

 nished me with the amount of water in the emery. 



It requires a continued red heat to drive out all the water, a 

 circumstance which is true for a number of minerals, particu- 

 larly for those containing a large amount of alumina, as dias- 

 pore and the micas, which will be spoken of in this paper. 



The powder, of which the water has been estimated, was 

 next submitted to levigation in a large agate mortar placed on 

 a surface of glazed paper; and when completed it was carefully 

 detached from the mortar, placed in a platinum capsule, heated 

 gently to drive off any hygrometric moisture, and weighed. 

 The increase of weight furnished the amount of silica taken 

 from the mortar. 



The levigation of one gramme was accomplished in two 

 operations, each requiring about twenty minutes; and by using 



