Condensation & Rarefaction of Aire 523 :- 
out. Any process to cool the air must be re- 
tarded by the condensation of part of the vapour 
it contains. Suppose for instance that a portion 
of the atmosphere contained 5 of its weight of 
aqueous vapour, and that 2 of this vapour were 
condensed by 50°of cold ; that is, .1_ of the whole 
elastic mass was converted into water; then the heat 
given out would be sufficient to raise the tempera- 
ture of the remaining mass of air and vapour 6 or 
8°, which sufficiently accounts for the small 
difference observed in the results upon warm 
vapoury air and cold dry air. Hence vapour, 
far from producing the change of temperature 
in question, tends to diminish the effect. 
If any doubt remained with me respecting 
the veal change of temperature that takes place 
in the operations related above, it was com- 
pletely removed by the results of the two fol- 
lowing experiments. 
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EXPERIMENT 10. 
Inclosed a small graduated glass tube of 4 of an » 
inch internal diameter, & 10 inches long, witha 
short column of mercury init, in the large receiv- 
er; the tube was sealed at one end and open at 
the other, so that a portion of air of given ca-. 
pacity was confined by the mercurial column, 
