92 PHYSICS. 
ether will immediately rise to the top, there becoming partly converted into 
vapor. The mercury will by this means be depressed, the depression being 
produced by the tension of the ether vapor, and being in all cases greater 
than what would prevail in the presence of a vacuum above the mercury. 
If the tube be depressed still more in the mercury of the lower tube, the 
height of the mercury will remain unchanged, while if air were present it 
would increase, owing to its continued compression of the gas. The more the 
tube is depressed, the more the quantity of fluid ether increases, and the 
vapor is consequently condensed, not compressed ; and this may be con- 
tinued so far as to exhibit an entire condensation of the vapor, provided that 
no air be present. If the pressure be diminished by elevating the tube, the 
vapor will again be formed. 
If vapor be contained in any space unequally heated in different places, 
the tension of the vapor in the whole space will be the same as in the 
coldest part, as may be shown by means of the apparatus represented in 
jig. 18. Let the bulb, a, be half filled with ether, and this brought to boil ; 
after ebullition has continued long enough to drive all the air out of the 
bulb and the tube connected with it, quickly immerse the lower open end 
of the tube, b, in a vessel, c, filled with mercury. On cooling the bulb a 
part of the vapor will become liquid, and the mercury will ascend in the 
tube, until the bulb has attained the temperature of the surrounding air. If 
the bulb be cooled to a still lower point, the mercury will rise higher, and, 
in fact, to such a point, as if not only the bulb, but even the entire tube had 
been greatly cooled. 
Various forms of apparatus have been employed to determine the 
expansive force of the vapor of water. This, however, at elevated tem- 
peratures and tensions, becomes very difficult. For moderate tensions, as 
those under 212° F., a form of apparatus may be employed, consisting of a 
vessel of mercury, in which are two glass tubes, the longer of which is a 
complete barometer, while in the shorter there is contained some water 
above the mercury, which is vaporized in the vacuum. The whole apparatus 
may be dipped in a vessel of water, and the latter heated, by degrees, from 
32° F. to 212° F. Both barometers will have the same temperature, and 
the elasticity of the watery vapor thus formed, may be obtained for any 
degree of temperature, from the ratio of depression in the vapor barometer, 
to the height of the mercurial column in the complete barometer. When 
this depression is reduced to 0° we have the true tension of the vapor. 
It is much more difficult to obtain the tension when the pressure exceeds 
several atmospheres. Quite recently, Arago and Dulong have instituted 
an extensive series of experiments, to obtain the elasticity of vapor at the 
highest pressures likely to occur. For this purpose they employed the 
apparatus represented in p/. 19, fig. 19, where c is a strong steam-boiler of 
plate iron, in which the steam is generated; f, the furnace; y, the grate ; 
t, the tube through which the steam escapes. In the cover two gun 
barrels, e and 7, are let in, open above and closed below, both being filled 
with mercury. The one descends below the water in the boiler, the other 
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