572 REGNAULT ON THE 
higher by many degrees than those of the air. For this purpose 
the globe is again placed in the sheet-iron vessel V V’ V", which 
can be easily done without displacing anything ; the barometric 
tubes remain outside the case, and the openings a and 6 in the 
sheet-iron vessel are closed by corks. The observations are 
sufficiently accurate when the water is not raised more than from 
10° to 15° above the temperature of the surrounding air, but 
beyond that limit the water of the globe begins to distil and 
condense in the tubes, and the observations become no longer 
exact. The series D, E were obtained in this manner. 
The methods which I have been describing are capable of 
great exactness ; the apparatus is easily arranged, and is equally 
applicable to the determination of the elastic forces of vapours 
which exist in air more or less rarefied. It is sufficient for this 
latter purpose, to replace the two barometers by a system of 
two tubes communicating with each other, as is represented in 
Pl. VII. at fig. 8. In this manner I determined the elastic force 
of the vapour of water in air, at the ordinary pressure, in order 
to ascertain whether this elastic force was really the same as 
in vacuo; but I shall reserve this for future publication. 
It is essential to the accuracy of the experiments, that the 
globe be most completely dried before the bulb is broken; this 
point is attained by producing a vacuum a great number of 
times with a good air-pump, and allowing, after each time, dry 
air to enter. I wished to satisfy myself whether it was not 
possible to dry the apparatus still more perfectly by heating the 
globe to a high temperature whilst the air was being pumped 
out, and by this means to obtain higher values for the tensions 
of vapour at 0°. 
The apparatus was modified in the following manner :—the 
glass bulb containing water was not placed in the globe, but in 
a small lateral tube mn, fig. 4, which is cemented on one side 
into the copper tube f, and communicates on the other with the 
tube containing the sulphuric acid and pumice-stone. The ap- 
paratus is dried in the ordinary manner by the air-pump ; but 
in order to render the drying more efficacious, the globe is sur- 
rounded with charcoal and heated to 300° or 400°; the baro- 
metric tube is at the same time slightly heated. The air-pump 
exhausts the air at last in a more complete manner than when 
the globe is not heated. The tube / is closed by the lamp. 
That equilibrium of temperature may be established in the baro- 
