ELASTIC FORCES OF AQUEOUS VAPOUR. 569 
II. The second series of experiments was made by means of 
the apparatus represented at figs. 1, 2,3. A globe A, containing 
about 500 cubic centimetres, encloses a small glass bulb entirely 
filled with water recently boiled. The globe is attached to a 
bent tube, which is cemented into a copper connecting piece 
having three arms, def. In the arm ea bent tube egh is ce- 
mented, and this is fused into the upper part of a barometric 
tube / a, which passes through the opening @ in the sheet-iron 
vessel VV’. Through the second opening 6 in the sheet-iron 
vessel a real barometer is passed, which dips into the same cup 
of mercury, U. To the third arm of the copper connecting piece 
is attached a tube communicating with the air-pump, but between 
it and the pump is interposed a tube M N filled with fragments 
of pumice-stone moistened with sulphuric acid, of about one 
metre entire length. 
The apparatus being thus arranged, a vacuum is produced a 
great many times, and after each time the air is admitted very 
slowly through the tube M N, which dries it as it passes. Having 
repeated this forty or fifty times, the globe and barometric tube 
may be considered quite dry; then a vacuum is made for the 
last time, and rendered as perfect as possible. The air-pump at 
my disposal could only be made with much difficulty, in the first 
experiments, to give a vacuum within 2 millimetres ; but having 
been cleaned, the vacuum was often obtained within less than 1 
millimetre. 
When the vacuum is as perfect as possible, the tube // is 
sealed by means of the lamp. The globe A is surrounded with 
melting ice, and after some time the difference in the height of 
the two columns of mercury is taken with the cathetometer; 
thus the elastic force of the dry air remaining in the globe at 
0° is obtained. The ice is then removed: the globe is heated 
by a few pieces of charcoal placed in a ladle with a bent handle, 
and the bulb is broken by the expansion of the liquid contained 
in it; the globe is again surrounded with melting ice, and after 
some time the difference in the height of the two menisci is ob- 
served. This difference, less that which existed between the 
heights of the two menisci before the fracture of the bulb, gives 
the elastic force of aqueous vapour for 0°. Care is taken to 
repeat these measurements a sufficient number of times, at inter- 
vals of about ten minutes, in order to be sure that the differences 
of height are quite constant. 
