io4 Sir Benjamin Thompson’s Experiments 
by water to a very confiderable degree ; and this I afterwards 
found to be the cafe with feveral other fubflances. 
Though the quality, at a medium, of the air furnifhed in 
this experiment was not quite fo good as that furnifhed in the 
two experiments lafb mentioned (viz. N° 13. and N° 14.), yet 
its quantity, in proportion to the quantity of water made life 
of, was greater than in either of them : it amounted to fome- 
thing more than one-eighth of the volume of the water. 
Of all the fubflances I had hitherto made ufe of in thefe 
experiments, raw filk had furnifhed the greatefl quantity of 
pure air ; or, to exprefs myfelf more properly, had caufed the 
water to furnifh the greatefl quantity ; but it appeared to me 
very probable, that fome other body might be found, - that 
pofTefl'ed this property in a flill greater degree than (ilk. Turn- 
ing this matter in my mind, it occurred to me, to make the 
experiment with the filky, or rather cotton-like, fubflance 
produced by a certain lpecies of the Poplar- tree, Populus 
nigra , very common in this country, and which, I believe, 
grows in England. I recollected that examining it fome time 
before, with a ditferent view (that of feeing if it might not be 
made ufe of with advantage, as a fubflitute for Eider down), 
and endeavouring to render it very dry, by expofing it in a 
china plate over a chafing- difh of hot embers, when it had 
acquired a certain degree of heat fmall parcels of it quitted the 
plate of their own accord, and mounted up to the top of the 
room. 
This convinced me of the time not only of its extreme 
finenefs, but aifo of the flrong attra&ion which fubfifts be- 
tween it and the particles of air ; and it now occurred to me, 
that thefe qualities not only render it peculiarly proper as a 
fubflitute of Eider down, for confining hear, but likewife are 
properties 
