ON THE COMPOSITION OF EMERY. 53 



The greatefr. number of trials was made with the infulated The knife al- 

 knife, which was always elearified contrarily to the chips ; wa > s contrar y* 

 but the furface of the wood where the chips were cut from 

 was very feldom electrified, and when it was it was always 

 but weakly fo, and of the fame denomination as that of the 

 weakeft of the other two. I have repeatedly found that if a Split wood has 

 piece of dry and warm wood is fuddenly fplit afunder, the the two ftates# 

 two furfaces which were contiguous are electrified, .one fide 

 pofitive and the other negative. 



X. 



On the Compofition of Emery. By Smith son Tennant, 

 Efq. F. R. S. (Ph. Tranf. 1802.) 



1 HE fubftance called emery, which, from its great hard- Emery has not 

 nefs, has been long ufed in various manufactures, for grinding JJJJ f^J^j 

 and poliftiing other bodies, has not, it appears, been hitherto 

 correctly analyzed. In books of mineralogy, it is confidered 

 as an ore of iron ; an opinion probably derived from its great 

 fpecific gravity, as well as from the iron which it frequently 

 contains. But, where this metal is moft abundant, it could 

 not be extracted from it with advantage, and ought rather to 

 be regarded as an impurity, as it does not contribute to pro- 

 duce the peculiar hardnefs for which this fubflance is diftin- 

 guiihed. In Mr. Kirwan's mineralogy, he mentions an exa- 

 mination of emery made by Mr. Wiegleb, from which he in- 

 ferred that 100 parts confified of 95,6 of filex, and 4,4 of 

 iron. Mr. Kirwan, however, juftly fufpects the correctnefs 

 of this account, and obferves that he had no doubt but fome 

 other flone was impofed on Mr. Wiegleb for emery. 



When powder of emery is boiled in acids, it becomes of a Emery powder 

 lighter colour, from the lofs-of part of the iron ; after which, S lves part of its 



• i r i r i i a • , lr0n t0 a«ds. 



it does not teem to undergo any further alteration. As acids 

 produce fo little effect on it, I expofed it to a pretty ftrong 

 red heat, with carbonate of foda, in a crucible of platina. 

 On adding water to the mafs contained in the crucible, the Soda by fufion, 

 greater part of the emery was found in powder ; having only dlfl ° lve ^ |° me 

 become of a light colour, from the extraction of part of the the emery. 



iron. 



