on the Production of Dephlogijlicnted Air , 107 
This experiment was repeated, and with nearly the fame 
refult; the total quantity of air produced being 41 f cubic 
inches, and its quality, at a medium, \a + $n— 1,26, or 274. 
To afcertain the relative finenefs of this poplar cotton, and 
the thread of raw filk as fpun by the worm, in order to make 
an eftimate of the furface of the former, I examined them 
both at the fame time under an excellent microfcope, when 
the diameter of the cotton, that is to fay, of a tingle threader 
fibre of it, appeared to be not more than half as great as the 
diameter of the filk, confequently its diameter was not more 
than -3 -Vtt part of an inch; for 1 have (hewn, in a former 
letter, that the diameter of a thread of filk, as fpun by the 
worm, is only TT f TT of an inch. 
The fpecific gravity of the cotton I found to be very nearly 
equal to that of water, confequently it is to that of filk as 
1000 to 172 4; its furface, therefore, is to the furface of an 
equal weight of raw filk in the compound proportion of 2 to 1, 
and of 1734 to iooo ; that is to fay, as 3468 to 1000. 
Now, as the furface of 30 grains of raw filk amounts to 
476 fquare inches, the furface of 30 grains of poplar cotton 
muft amount to 1651 fquare inches, which gives 55 fquare 
inches of furface for each grain in weight ; confequently the 
furface of the cotton made ufe of in the foregoing experiment 
(N 9 16.) did not amount to lefs than 6600 fquare inches (for 
120 grains, the weight of the cotton, multiplied by 55, gives 
6600) an enormous furface indeed for a body, whofe folid 
contents did not amount to quite half a cubic inch. 
From hence it appears evidently, that the quantities of air 
furnifhed by water, in the experiments with raw filk, and with 
poplar cotton, were neither in proportion to the quantities of 
thefe fubftances made ufe of, nor to the quantities of their 
P 2 fur faces. 
