594 On Evaporation. 
stance, and the aqueous atmosphere found at 
22°. By problem 2, at page 588 it would have 
-been determined at 214°, using the second column 
of grains inthe table.* . 
* On the subject of evaporation it may be considered 
as unpardonable not to advert to De Saussure’s valuable 
Essays on Hygrometry. 
That excellent philosopher determined, by a well 
conceived experiment, that dry air of the temperature of 
64° or 66°, imbibed aqueous vapour so as to increase its 
elasticity =4; of the atmospheric pressure; and that a 
cubic foot of such air required 11 or 12 grains of water 
to produce the effect. By the table above at page 560 
‘it appears the force of vapour at 61° = .54 = zy of 
29 .5 inches, nearly. It is probable this difference is 
occasioned in part at least by the want of perfect dry- 
ness in the air he operated upon, which caused the 
increase of elasticity to be less than otherwise,—It was, 
I think, unfortunate that he attached so much impor~ 
tance to and confidence in his hygrometer; and that he 
adopted the theory of chymical solution of water in air, 
contrary to the facts he discovered, which seemed more 
reconcileable to the notion of aqueous vapour being a 
distinct elastic fluid. Indeed he is forced to acknow- 
ledge in the 1st. chap. of his Essay on the Theory of 
Evaporation, that in the ordinary temperature of the 
atmosphere, aqueous vapour is formed in the first ins 
stance a distant elastic fluid, and after zt has been con- 
verted into an elastic fluid, it is dissolved by the air; 
** Je crois quil ne la dissout que lorsque V’action du 
* feu l’a convertie en vapeur elastique.”- Now if it 
can for a moment exist independently under the pressure 
