70 
ON THE PHENOMENON OF DEW. 
atmospheric veins diverge outwards to the margin. At the point of 
each vein there is a slight indentation in the margin, and in this the 
dew-drop is suspended, showing that it is attracted by some action of 
the vein of the leaf, rather than by the more prominent parts of the 
margin. 
Dew begins to be formed as soon as the solar beams are withdrawn, 
and is increased till sun-rise, when it is gradually exhaled away. That 
the dew-drops are condensed vapour, is self-evident; but whether this 
vapour descends from the atmosphere, or is evaporated from the earth 
by the radiation of the sun’s heat, united w’ith that subterranean heat 
which is ever escaping from the surface, (together producing, by the 
escape of heat, that extreme cold on the surface,) is not so evident, 
though, perhaps, the preceding observations, if faithful, may assist 
those, who contemplate the phenomenon, to arrive at a pretty sound 
conclusion. The evaporation from the earth is often beautifully visible 
on damp ground on serene evenings in summer and autumn, lying like 
a thin cloud on the steaming surface after a hot day, and, doubtless, is 
ever rising whether visible or not. 
But there are other condensations of atmospheric moisture which 
should be considered in connection with the foregoing observations on 
dew, as they are phenomena perfectly distinct from each other. Water 
is soluble in air, and unites with it so intimately, that its transparency 
is not in the least diminished, or it is buoyant in the atmosphere, in a 
vesicular state, and takes the appearance of mist or clouds. A change 
of the temperature of the air produces a change of the condition of the 
water floating in it; that which is in a state of perfect solution becomes 
vesicular, and that which is in the form of clouds is condensed into 
larger drops, and descends as rain. So all solid bodies on the earth 
being often colder than the air, attract from the latter both its 
heat and aqueous particles, condensing the water into large drops on 
the surface, which coalescing run in little streams to the ground. 
During a season of frosty w r eather, all solid bodies, as stone, marble, or 
metals, are cooled down according to the intensity of the frost, and, on 
a return of warmer weather, these solid bodies w r ill be continually 
covered with moisture till their temperature becomes equal to that of 
the air. This circumstance is frequently witnessed in summer, as well 
as in winter. All solids, as the stones and pavement in the streets or 
around buildings, become cool during nights ; and such as lie out of 
the morning rays of the sun, will continue to attract moisture till nine 
or ten o’clock, or till their temperature is equal to that of the air. 
All these instances of the condensation of vapour are the effects of 
that general law of nature, by which all fluids tend to an equilibrium. 
