Condensation of Atmospheric Humidity on Solid Surfaces. 87 
with in a state that can, with any attention to accuracy, be 
termed dry ; more commonly they are in some degree damp. 
Besides, their temperature, when laid on the ploughed field, 
would, in all probability, be somewhat lower than that acquired 
by the surface of the bare earth, exposed to the direct influence 
of the sun’s rays ; and would, consequently, be lower than that 
of the aqueous vapour issuing from it. Independently, there- 
fore, of their hygroscopic property, and of their mechanical ope- 
ration, afterwards to be adverted to, they might thus acquire 
an accession to the moisture which they previously contained ; 
and portions of this moisture being carried off by the contiguous 
air, their temperature would, in a few minutes, be reduced to 
that of an adjacent field of grass, and it might be in certain cir- 
cumstances even lower. 
It is generally admitted, that when the temperature of a body 
is considerably higher than that of the contiguous air, it will lose 
heat, both by conduction and radiation, or by some process equi- 
valent to the latter. But as even a current of air cannot cool a 
perfectly dry body, below its own temperature, when any body 
is found to be colder than the air, the question to be determined 
is, Whether the loss of heat is to be attributed to evaporation, or 
to some process equivalent to that which has been termed radia- 
tion ? 
In the case referred to, the sameness of temperature in the 
grass, the hay, and the wool, is to be attributed to their being 
equally bad conductors of heat, and equally capacitated for sup- 
porting evaporation ; and that not only from one exterior or up- 
per surface, but from numerous interior surfaces, to which the 
air had access, and from which the sun’s rays were more or less 
perfectly excluded. The naked soil, on the other hand, though 
comparatively a dense solid, and a good conductor of heat, has 
but one evaporating surface ; while its dark or nearly black 
colour, enables it to absorb a greater proportion of the sun’s 
rays, and convert them into heat of temperature, than bodies of 
a white, pale-yellow, or green colour. Hence the temperatures 
of the grass, hay, and wool, were somewhat less than that of 
the naked soil ; relatively less heat being abstracted from the 
latter, by the process of evaporation, than it acquired through 
the influence of the sun’s rays. 
