ELASTIC FORCES OF AQUEOUS VAPOUR. 561 
method in my experiments on the capacity of bodies for heat, 
and in those which I made upon the expansion of elastic fluids ; 
it will likewise be adopted in the researches which I now pub- 
lish on the elastic force of vapours. 
I. Nearly all the experiments undertaken for the purpose of 
determining the elastic force of aqueous vapour at low tempera- 
tures, have been made by means of two barometers plunged into 
the same cup of mercury; into one of these barometers a small 
quantity of water was introduced, which ascended into the ba- 
rometric vacuum. The difference in the heights of the two ba- 
rometers, placed in identical circumstances, represented the ten- 
sion of the aqueous vapour for that temperature at which it ac- 
tually was. The great defect of this mode of operating consists 
in the difficulty of determining exactly the temperature which 
corresponds to the tension observed. The greater number of 
experimenters who have followed this method within the limit 
of atmospheric temperatures, have contented themselves with 
placing a mercurial thermometer near the barometers, at the 
same height as the vacuum chamber, and observing the indica- 
tions ; these were presumed to denote the temperatures corre- 
sponding to the tension. M. Kaemtz carried out in this manner 
a long series of observations, extending over the space of two 
entire years, and he obtained the elastic forces of aqueous vapour 
from —19° to + 26° (Météorologie, vol. i. p. 290). The same 
mode of procedure has been employed by various authors for 
higher temperatures than those of the atmosphere. In this latter 
case the two barometers were placed in a glass vessel filled with 
water, the temperature of which was gradually raised. Dr. 
Dalton placed only the humid barometer in a second larger 
glass tube, closed at the base by a cork, through which the ba- 
rometer-tube passed; he filled the space between the tubes with 
water raised successively to different temperatures. 
This method is not capable of very great accuracy for it is 
impossible to keep a column of liquid of a certain height at a 
uniform temperature, without constantly agitating the liquid ; 
and in the experiments of Dalton there exist no means of ren- 
' dering the temperature stationary for a length of time sufficient 
to allow of equilibrium of temperature being established between 
the column of mercury and the surrounding liquid. 
I made some experiments in this manner for the purpose of 
ascertaining within what limits the results were exact. 
