on the mechanical Expanfion of Air. < .« 
and inftantly devaporated : whence the very quick recipro- 
cations of the pifton ; and that, though the cylinder itlelf is 
always kept as hot as boiling water, that is, as hot as the 
fteam was previous to its devaporation. 
Something very fimilar to this is often feen at the com- 
mencement of thunder-ftorms ; a fmall black cloud at fir ft 
appears, in a few minutes the whole heaven is covered with 
condenling vapour, and the accumulation or efcape of eleCtric 
matter feems to be rather the confequence than the caufe of 
this fudden and general devaporation. 
A fecond curious circumftance of aerial devaporation is, 
that when the particles of aqueous vapour begin to approach 
each other by the diminution of their heat, they do not gene- 
rate water exactly in proportion to fucli diminution of heat ; 
but the condenfation proceeds further, and not only a greater 
quantity of water is produced, but alfo a quantity of heat is 
fet at liberty along with this excefs of devaporation, and the 
atmolphere becomes warmer than before the beginning con- 
denfation. This excefs of devaporation beyond the cold which 
produced it, is probably owing to the acquired momentum of 
the aqueous particles towards each other at the beginning 
of their condenfation, which carries them ftill nearer each 
other ; and to the fmall moleculae at ftrft formed, poflefllng a 
greater attractive power over the uncondenfed vapour in their 
vicinity, and thus prefting out more of the latent or combined 
heat. 
Conclufon . 
i. When a fmall portion of air, fuppofe a few acres, be- 
comes fuddenly contracted into a lefs compafs, either by inci- 
H 2 dental 
