C 2r + ] 
florted in their fhape, as is remarkably the cafe with 
wood, and, I believe, all vegetable fubftances. 
In examining mineral l'ubftances, I found that 
my information, mentioned above, was juft. Coals 
are very much inlarged in their dimenfions by char- 
ring ; but the experiment muft be made with great 
care, to judge of this circumftance ; for, unlefs the 
operation be very flow, the coal will retain nothing of 
its former fhape, having been made in fome mealure 
fluid by the heat. The infide of all pieces of pit 
charcoal is full of cavitities, and there is generally a 
very large one in the center of every piece; l'o that, 
the dilatation is 'nothing like the extenflon of fibres; 
but is produced by the elafticity of the new formed 
vapour, in forcing its way out, while the fubftance 
is foft. 
With refpeCt to the main object of my inquiry, 
I prefently fatisfied myfelf, that the conducting power 
of charcoal depends upon no other circumftance than 
the degree of heat, that is applied in the procefs of 
making it. I had not fufpeCted this; but numberlefs 
experiments clearly proved it. Taking an iron pot 
filled with fand, and putting into it pieces of wood, 
cut out of the fame plank, marking them, and care- 
fully noting their p'aces in the pot, I always found 
that thole pieces came out the beft conductors, that 
had been expofed to the greateft heat. The refult 
was the fame when I made coals of bits of wood, 
palaced one above another, in a gun barrel, one end 
of which was made red hot, and the reft gradually 
cooler and cooler. 
Taking pieces of charcoal that conducted very 
imperfectly, or not at all, I never failed to give them 
the 
