On the Expansion of Elastic Fluids by Heat. 595 
ESSAY IV. 
On the Expansion of Elastic Fluids by Heat. 
The principal occasion of this essay is another 
on the same subject by Messrs. de Morveau 
and du Vernois in the first vol. of the Annales 
de Chimie. It appearing to them that the 
results of the experiments of De Luc, Col. 
Roi, de Saussure, Priestley, Vandermonde, Ber- 
of the atmosphere, why may it not continue to exist in 
that state? ” 
His table of the weight of aqueous vapour in a cubic 
foot of air at different degrees of the thermometer, 
being derived from experiments with his hygrometer, 
except the standard one of 66. (15° Reaumur), is far 
from accurate ; and the inaccuracy increases with the 
distance from the standard, which, as has been observed, 
appears to be nearly correct: in the higher. temper. 
atures he makes the water dissolved too little, and in 
the lower temperatures too much,—He says (§ 93) that 
the lowest he has seen the hygrometer in the open air, 
is 40; and that it indicated a reduction of temperature 
in the air amounting to 34° .7 (78° of Fahr.) was neces- 
sary in order to deposit dew, This observation alone 
is sufficient to render his hygrometer suspected; for, 
few who have attended to the formation of dew will 
admit the probability of so large a reduction being ne- 
‘cessary in any climate or season: I believe it rarely 
‘requires 40° reduction in temperature in any part of the . 
world to produce the effect, 
