95 
and the Mode of its Communication. 
to paint the body of the instrument three or four times over, in 
order to cover the polished metal so completely as to prevent 
its shining through the paint; this of course occasioned the 
surface of the metal to be covered with a thick coating of size, 
which, no doubt, affected very sensibly the results of the ex- 
periment, and rendered it impossible to determine, in a satis- 
factory manner, what the effects really were, which were pro- 
duced by the different colours used in the two experiments. 
Exper. No. 11. With a view to throw some more light on 
this interesting subject, having washed off the paint from the 
instrument No. 2, I now rendered its sides of a perfectly deep 
black colour, by holding it over the flame of a wax candle ; and, 
repeating the usual experiment, the results were as follows. 
Time of cooling through 
the standard interval. 
The instrument No. 1, naked - 55-2- min. 
No. 2, blackened - - 36-i min. 
In order to ascertain the quantity of matter which composed 
this black covering, I weighed a small piece of clean and very 
fine linen ; and, having wiped off with it all the black matter 
from the body of the instrument No. 2, in such a manner that 
the whole of it remained attached to the linen, I weighed it 
again, and by that means discovered that the whole of this black 
substance, which had so completely covered the sides of the in- 
strument (a surface of polished brass 3=50 superficial inches) 
that the metal did not shine through it in any part, weighed no 
more than ^ of a grain Troy. 
How this very thin covering, which, if the specific gravity of 
the black matter were only equal to that of water, would amount 
to no more than ^ of an inch in thickness, could expedite 
