Condénsation  Rarefaction of Air. 52% 
The phenomena in the two last experi- 
ments can be explained only on the fol- 
lowing principle: The air in the receiver and 
in the manometer is subject to a like degree 
of rarefaction and condensation in those ex. 
periments, or very nearly so, When the equi- 
librium of heat in the air is disturbed by the 
operations of condensation and rarefaction, it is 
restored in the manometer instantly, by reason of 
the contiguity of the glass to the air; but in the 
large receiver it requires a sensible time of 10 
seconds or more to restore the equilibrium 
throughout the whole internal capacity. It is 
this restoration that increases or diminishes the 
elasticity of the air confined in the receiver, and 
thereby causes the retrogradation of the mercurial 
column. Now I have found by former experi= 
ments that.a change of 50° in temperature effects 
a change of 1. nearly, in the capacity or bulk 
of air. It follows therefore that in the case 
of restoring the equilibrium in condensed air, 
about 50° of cold is produced; and in letting 
in air to an exhausted receiver something more 
than 50° of heat is produced. The small differ. 
-ence seems to arise from this, that the conden- 
sation of vapour in the former: case diminishes 
the effect, and in the latter, if any there be, 
increases the effect, that would arise from Ope-: 
rating upon purely dry air. 
