[ 22 3 ] 
ing electricity, I fufpeCted that thefe two different 
fubffances might alfo agree in their readinefs to ex- 
pand by heat. Mr. Smeaton was fo obliging as to 
aflift me in my attempts to afcertain this circum- 
ftance, by the application of his excellent pyrometer. 
Though we could not make the experiment with all 
the exa&nefs that we could have wifhed, yet the re- 
fult of near thirty trials was uniformly in favour of 
the greater degree of expanfion, by heat in the 
charcoal, than in wood of the fame kind, as we 
imagined, out of which it was made. In general, 
the expanfion of the charcoal was about double to 
that of the wood. 
It is evident that a certain degree of heat makes 
wood and charcoal expand, and alfo that a greater 
degree of heat makes them contract. I wifh we 
had an inftrument to afcertain the precife degree of 
heat, at which the expanfion ceafes, and the con- 
traction begins ; and whether the two effects be pro- 
duced by the fame gradation. 
In the courfe of thefe experiments on charcoal, I 
met with afubftance, the conducting power of which 
i$ fingular, and exhibits a beautiful appearance. la 
order to fee what would remain after burning a 
quantity of turpentine in a glafs tube, I covered it 
with fand, in a crucible, in the fame manner in 
which I ufed to make charcoal j and, after letting it 
continue a fufficient time, in a very hot fire, and the 
flame had long ceafed, I examined the tube, and 
found that it had been melted ; but, inftead of any 
thing like charcoal, or the leaf!: blacknefs, I obferv- 
ed that the tube was uniformly lined with a whitifh 
gloffy matter, that I could not fcrape off. Upon 
trying 
