Non-conduéling Power of Charcoal. 183 
fulating effect of charcoal. The experiments which I made 
on this fubject are as follows :—Having taken from the fame 
box, two of ‘Wedgewood’s  pyrometrical pieces perfectly 
fimilar, I put one, which I ‘hall call A, into a crucible 
filled with pure filiceous fand, well dried. The crucible 
was cight decimetres in height, and fix decimetres in 
diameter at the top. The piece of clay was placed in the 
middle, and the cover was luted on. The other piece B 
was placed in a covered crucible of the like fize, but filled 
with charcoal duft which had been previoufly expofed to 
ared heat. The two crucibles were placed clofe to each 
other on the grate of a large fufing furnace, and expofed to 
the action of the heat for about three quarters of an hour. 
When the crucibles had cooled, the piece A was taken from 
the fand, and applied to Wedgewood’s pyrometrical {eale : 
it had experienced a contraétion of 89 degrees. The piece 
B, when taken from the charcoal, ftood at 60°25 degrees. 
It had affumed a greyifh tint, but without any appearance 
of incruftation. It thence refults that the tranfmiffion of 
heat through fand is to that of heat through charcoal as 
4 to 2*, ; 
The more the difference was ftriking, the more it was ne- 
ceflary for me to employ precautions, that I might not be 
deceived by foreign circumftances. It was poffible that the 
_® This is not ftated with that accuracy ufually difplayed by Guyton 
I¢ leads to a conclufion, that the refult would» haye been the fame had the 
pieces been continued in the furnace for three hours, three days, or three 
weeks, inftead of aZout three quarters of an hour, which is ao ynwarrant- 
able inference. This experiment only proves, and that is a matter of 
fome confequence, what might have been fairly inferred from the known 
properties of charcoal, that to bring bodies furrounded with that fubftance 
to the fame degree of heat with others furrounded with better conduétors, 
a longer time muft be employed. The not attending to this circumftance is 
the caufe why chemifts, able ones too, often fail in difficult reduétions 
where charcoal is neceflarily put round the ore to take up the oxygens 
Epit. 
N4 pyto- 
