Mr. Daniell on Evaporation. 49 
By enclosing in a glass receiver, upon the plate of an air-pump, 
a. vessel with sulphuric acid, and another with water, and by pro- 
perly adjusting the surfaces of the two, it is easy to maintain, in 
the included atmosphere of permanently-elastic fluid, an atme- 
sphere of vapour of any required force; or, in the usual mode of 
expressing the same fact, the air may be kept at any required de- 
gree of dryness. The density of the air, in such an arrange- 
ment, may, of course, be varied and measured at pleasure. Now 
there are three methods of estimating the progress of evaporation 
in such an atmosphere: the first, and most direct, is to weigh the 
loss sustained by the water in a given time; the second, to mea- 
sure, by a thermometer, the depression of temperature of an eva- 
porating surface; and the third, to ascertain the dew point, by 
means of the hygrometer. 
Experiment 1. 
The receiver, which I made use of, was of large capacity, and 
fitted with a hygrometer. I placed under it a flat glass dish, of 
74 inches diameter, the bottom of which I covered with strong 
sulphuric acid.. The glass bell but just passed over it, so 
that the base of the included column of air rested everywhere 
upon the acid. In the centre of the dish, was a stand with glass 
feet, which supported a light glass vessel of 2°7 inches diameter, 
and 1:3 inches depth. Water to the height of an inch was poured 
into the latter, the surface of which stood just three inches above 
that of the acid. A very delicate thermometer rested in the water, 
upon the bottom of the glass, and another was suspended in the 
air. It may be necessary to observe, that the sides of the vessel 
were perpendicular to its bottom, which was perfectly flat. The 
height of the barometer was 29°6, and the temperature of the 
water 56°. In twenty minutes from the beginning of the experi- 
ment, the hygrometer was examined, and no deposition of mois- 
ture was obtained at 26°. 
This being the greatest degree of cold which could be conye- 
niently produced by the affusion of ether, the experiment was re= 
peated, with a contrivance which admitted of the application of a 
Vou. XVII. E 
