50 EMERY MINE OF CHESTER. MASS. 



alumina. Some of the alumina found in the analysis is associ- 

 ated with the above ingredients to form associate minerals 

 which have been fully studied. This last will serve to explain 

 why it is that emeries having the same amount of alumina may 

 have different degrees of effective hardness. Thus : Nos. 9 

 and 4, both Kula emeries, containing about the same amount 

 of alumina, have effective hardness in the proportion of 40 to 

 53; but it will be seen that No. 9 contains 9.6 per cent, of 

 silica, which doubtless appropriates a portion of the alumina, 

 thus reducing the alumina attributable to corundum ; so that 

 were it possible to ascertain the exact amount of corundum 

 present in 9 and 4, it would doubtless be in proportion to their 

 effective hardness. So again, if we compare Nos. 8 and 1, the 

 effective hardness will be found in the proportion of 42 to 57, 

 while their amounts of alumina vary only as 60 to 63 ; but if 

 we regard the amount of water in the two, it is as 5.6 to 1.9, 

 much of this water coming from diaspore that is intimately 

 mixed with the corundum; and in several specimens I possess 

 the two minerals shade into each other so completely that it is 

 impossible to tell where one begins and the other ends. The 

 above facts were all well examined when my first memoirs 

 appeared on this subject, which accounts for the following 

 remark then made : " Those emeries which contain the least 

 water, every thing else alike, are the hardest, as instanced by 

 that from Kulah, notwithstanding the quantity of iron it con- 

 tains. The silica existing in emery is most often in combination 

 with alumina or the oxide of iron, or both ; for this reason we 

 must not always regard the quantity of alumina as an indication 

 of the quantity of corundum in emery." 



In concluding this part of the subject I would state that 

 while I do not consider my opinions infallible in this matter, 

 still all my experience and research gathered from such varied 

 sources point to the conclusion that emery is a mixture of 

 several minerals, principally corundum and magnetic oxide of 

 iron, the former being the effective agent in the mechanical 

 abrasion to which it is applied ; the oxide of iron is not to be 

 considered as an unimportant ingredient, it serving by its 

 presence to destroy to some extent the harsh cutting action of 

 the corundum. 



