608 REGNAULT’S HYGROMETRICAL RESEARCHES. 
filled with dry air under the pressure of the atmosphere, whilst 
the balloon is surrounded by melting ice. The mercury is 
brought exactly to the line of level on the tube pg, whilst the 
balloon still communicates freely with the air; lastly, the tube 
if, fig. 1, is sealed over the lamp. The ice surrounding the bal- 
loon is removed, and the vessel VV’ filled with water, which is 
successively raised to higher temperatures. At each observation 
the mercury is restored to the mari on the tube pg, the water 
of the vessel is kept at a stationary temperature, as stated in the 
preceding memoir, page 565, the difference of height in the two 
columns of mercury is determined, and the barometer noted. 
The apparatus thus acts as an air thermometer, and it is ascer- 
tained that its indications correspond exactly with those of the 
mercurial thermometer immersed in the same water. 
The water is then removed from the vessel, the bulb broken 
by bringing some incandescent charcoal near to the bottom of 
the balloon, the water replaced in the vessel, and the same series 
of observations recommenced on the air saturated with humidity 
which had been previously made on dry air. The difference of 
the elastic forces found in the two cases, and corresponding with 
the same temperature, is evidently equal to the tension of the 
aqueous vapour saturating the air at the same temperature. 
The tension of the aqueous vapour in the air cannot be deter- 
mined with the same precision as in vacuo; it requires greater 
precaution, and the error committed relative to the expansion 
of the air is added to that which occurs for the elastic force. 
Moreover, the maximum tension of the vapour occurs instanta- 
neously i vacuo, whereas it requires a somewhat long time to 
be established in the air. It is necessary to ascertain with cer- 
tainty, by observations repeated at distant intervals, and for one 
and the same stationary temperature, whether the tension does 
not continue to increase. 
For greater certainty, a first series of experiments was made, 
by raising successively the temperature of the water of the ves- 
sel, so that the air had to dissolve a further quantity of vapour ; 
a second series was then made, in which, on the contrary, the 
temperature of the water was gradually lowered; the air then 
deposited a portion of the water which it had previously held in 
solution. The elastic forces of the vapour at the same tempe- 
rature should be found identical in these two series. 
The following tables contain the results obtained by this me 
