Condensation of Atmospheric Humidity on Solid Surfaces . 83 
a thermometer with wool, Sec. or to ascertain the temperature of 
such substances, after they have been exposed, for a certain time, 
to the open sky, after sun-set, or to the direct influence of the 
sun’s rays. 
It must be admitted, however, that all such experiments are 
necessarily and in no small degree defective ; and whatever the 
results may have been, they can never prove that which they 
were intended, and have been supposed to establish. When 
it is received as a rule, that u no more causes are to be admitted 
than are sufficient to account for the phenomena,” it must also 
be admitted, that, when two or more causes are immediately 
operative, the effect cannot be attributed to any one or more 
of these, to the exclusion of the rest. In the experiments re- 
ferred to, all the substances made use of, such as wool, cotton, 
silk, lint, down, saw-dust, straw, Sec. are not only bad conductors 
of heat, but of that description of substances which, according 
to circumstances, absorb, or give out moisture to the atmosphere, 
with the greatest facility. Admitting, then, that, in certain cir- 
cumstances, bodies, at the surface of the earth, did radiate their 
heat, so as to become colder than other bodies in contact with 
them, when experiments are brought forward to prove this effect 
of radiation, it is indispensably requisite to shew, either that eva- 
poration was in no degree operative, or that its effects were in no 
degree proportionate to the observed decrement of heat. In this 
point of view, by far the greater number of Dr Wells’s experi- 
ments seem altogether unsatisfactory, in as far as they were in- 
part with a portion of its moisture ; but, sooner or later, a period arrives, when it 
ceases to become drier. If, at this period, however, we bring it into a body of 
air that is considerably drier, but of the same temperature, and still keep its own 
temperature equally above that of the air, we find that it gives out an additional 
quantity of moisture. If, lastly, we replace it, other circumstances being the 
same, in a body of damp air, we find that it regains a certain quantity of mois- 
ture. Is there not here a certain resemblance to what takes place when the tem- 
perature of a body is diminished by the process of evaporation ? In the one case, 
there is a loss of heat until an equilibrium is established, that is, when as much 
heat is supplied by the air, as is carried off by the aqueous vapour. In the other 
case, there is a loss of moisture until an equilibrium is effected, that is, when as 
much moisture is absorbed as at the same instance escapes with the portion of air 
that is rarified ? 
F 2 
