. Uygrometry . 395 
more and more as the evaporation increafed by more heat, indi- 
cated that increafe in the quantity of vapour, diftindl from the 
expan fions of the fluids', and that each of thofe inftruments 
remained alfo fixed in that ftate of oppofite changes, by every 
fufficiently durable change of temperature. The firft obferva- 
tion of that phenomenon would have furprized him, as it 
did me, and it would have induced him alfo to try fome other 
of thofe flips ; and by finding the fame phaenomenon with 
every one of them, he would have been convinced, that it was 
a real law of moifture. Laftly, if he had known Mr. James 
Watt’s obfervations on the great drynefls of the fleam of boil- 
ing water , as long as it remains in a fpace as warm as the 
water that produces it (a condition always underftood when the 
general laws of evaporation are the object of inquiry) ; admit- 
ting with me, as it follows from his own theory, that luch fleam 
is no other than the fame kind of vapour thus far mentioned, 
only rendered capable of a greater denjity by more heat j he 
would then have feen the importance of that hygro/copic law 
from its great extent. I have hardly any doubt, that extreme 
drynefls would reign in a clofe place fimilar to Papin s dige/lor, 
if there were a fufficient fpace above the water and a red 
heat ; though that fpace would be filled with vapour to the 
maximum. This relates only to the hygroflcopic law here in 
view, on which, in the fuppofed cafe, I do not think we could 
have had any controverfy. 
70. Laftly, let us fuppofe, that in order to try the efFe£t of 
moifture on the fubftance he had firft chofen, or on fome other 
of the fame kind of fubftances, but taken lengthwifle , M. de 
Saussure had happened to try firft one of thofe which, 
ufed in that manner, have a great retrogradation, as goofle-quill 
and deal', and that, after having previoufly obferved it in the 
F f f 2 open 
