94 PHYSICS. 
placed, about a foot in length, surrounded by the glass covering m, itself 
filled with an appropriate fluid. Upon the horizontal ground edge of the 
vessel, c, lies a small board, ¢, through which passes the divided vertical rod, 
r. Before introducing the tube, g, into the vessel, it must be entirely filled 
with mercury, so that after immersion it may remain filled with mercury, 
and contain no air-bubbles. Now introduce a smal] glass bulb, fiiled with 
water, and with the opening melted together, into the tube, g; it will rise 
to the top, and on the mercury being heated, will burst by the expansion of 
the water. Vapor of water will immediately form in the upper part of the 
tube, g, and the mercury in it will smk. When, by continued application 
of heat. all the water becomes vaporized, the weight of the vapor will be 
known. provided that the volume of water in the bulb had previously been 
ascertained. The volume of the water is ascertained by the divisions on 
the tube, g ; its temperature by the thermometer; and then its tension by 
the graduated rod, r._ This latter is pushed down until its lower extremity 
touches the mercury in the vessel, c; the slide, v, is brought to an equal 
height with the surface of the mercury in the tube, by which means the 
height of the latter is ascertained. This, deducted from the barometer 
pressure, gives the tension of the vapor. . 
From the now known weight of a given volume of steam, which at a 
known temperature exerts a known pressure, the weight of any volume of 
vapor can be ascertained. As we have previously ascertained the density 
of the air to be = 0.001299, we can ascertain the weight of equal volumes 
of air and watery vapor at equal temperatures and equal pressures, and thus 
determine the ratio of density of the two. According to Gay Lussac, the 
density of steam is five eighths of that of the air. To determine the density, 
d, for other temperatures than those investigated, the following formula by 
P (1+ 1002) ; 
Gay Lussac may be émployed: d' = d= _~—- >, where d. is thedem 
760 (1 +aT) 
sity at 212°F., and a barometric pressure of 29 inches ; P, the pressure, and 
a, the coefficient of expansion, amounting, according to Gay Lussac, to 
0.00375. It is, however, assumed that vapors, like gases, follow the law of 
Marriotte to the maximum of tension. 
The density of the vapors of various other liquids has been investigated 
by Dumas, Gay Lussac, and others. 
Vapors are condensed by pressure and by cold ; nevertheless a vapor can 
be compressed without being at the same time partly condensed, only when 
it is not saturated. Hence we are led to the conclusion that even the so-called 
permanent gases are really vapors which are far from their point of satura- 
tion. Davy, and particularly Faraday, have succeeded by means of great 
cold and pressure in condensing into liquids, and even solids, gases which 
had previously been considered permanent. The method employed consisted 
in condensing the gases by their own pressure, for which purpose an instru- 
ment was used similar in its principle toWollaston’s eryophorus for producing 
artificial ice, but rather more simple. 
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