upon the Minimum Temperature of the Night. 163 
the atmosphere, under the vaporous form, is entirely regulated 
by temperature, and totally independent of chemical solution, 
the hygrometric condition of the air must, at all seasons, nearly 
correspond with complete saturation at the minimum tempera- 
ture ; more especially from the summer to the winter solstice, 
a period of the year when the accumulated moisture is return- 
ing back, by the decline of temperature, from the vaporous to 
the liquid state. In the present paper, however, it is not so 
much my design to illustrate this point, as to endeavour to shew 
that the hygrometric condition of the atmosphere exerts a con- 
siderable reaction upon the temperature by which it is induced ; 
the humidity of the air producing, in this respect, upon its tem- 
perature, an effect analogous to that of the fly-wheel of the ma- 
chine, which regulates and controls the operation of the moving 
power by which it is itself kept in motion. 
Every person who is at all acquainted with the general doc- 
trines of caloric, is aware of the fact, that liquids, during their 
transition to the state of vapour, always absorb a large propor- 
tion of the matter or substance of heat. This heat is not dis- 
coverable, in its combined state, by any of the ordinary means 
which we employ to detect the presence and intensity of that 
subtile agent. In the case of the vapour of water, it exists in 
union with the aqueous particles, in what is called a latent or 
concealed state ; but the characteristic energy of its nature is 
only suspended or neutralized^ not destroyed. The vapour, 
with which it is united, cannot recover the liquid form, until the 
watery particles have been disengaged from the igneous fluid, 
which being thus separated, resumes its wonted activity, and 
produces upon contiguous bodies, with which it enters into 
new combinations, the usual indications of its presence and ope- 
ration. Thus, when a quantity of common air, completely sa- 
turated with humidity, at any particular temperature, is made 
to suffer sudden condensation, the extrication of heat from the 
compressed vapour is not only perceptible by the senses, but 
sufficiently great to set fire to inflammable substances, placed 
within the sphere of its activity. The effect may be ascribed in 
part, no doubt, to the compression of the air, and the change 
of capacity for caloric, which it undergoes by condensation ; but 
it seems to be chiefly owing to the transition of the vapour to 
l 2 
