[ 218 ] 
not be meafured with fufficient accuracy, in thefe 
fmall pieces. To make them perfed condudors, 
they were reduced to about -fg. in weight, and JL 
in length. 
A variety of circumftances led me to conclude 
that the caufe of blacknefs, and of the conducing 
power in charcoal, is the oil of the plant, made 
empyreumatic, and burnt to a certain degree. I 
therefore conclude that thefe properties are fome way 
connected with that part of the inflammable prin- 
ciple, otherwife called phlogifton, that is fixed and 
united to the earth of the plant, when the union is 
ftrengthened by an intenfe heat. 
The fand, with which I covered the fubftances 
that I converted into coals, and alfo the pipe clay 
which I fometimes put over them, contraded a 
blacknefs like charcoal, and would often condud 
pretty well. Sometimes they would condud a 
fhock. This muft have been owing to the oil they 
received from the fubftances out of which it was 
expelled by the heat. In the experiment of the 
gun barrel filled with pieces of wood, mentioned 
above, the uppermoft pieces were not in the leaft 
burnt. They could hardly have been hot ; yet, 
having contraded a fuperficial blacknefs, from the 
vapour of the oil expelled from the piece below 
them, they would even condud a fhock, though not 
in the moft perfed manner. 
Sometimes thofe fubftances that had no phlogifton 
themfelves, but received it in confequence of being 
placed in the neighbourhood of other bodies out of 
which it was expelled, would not condud immedi- 
ately ■, but would be made to do fo by being expofed 
to 
