During summer, when the weather is sultry, and the sky assumes that
beautiful blue tinge so entirely its own, dew is formed in the greatest
abundance, owing to the phenomena which are requisite for its deposition
being then most favourably combined. It was long supposed by naturalists
that this precipitation depended on the cooling of the atmosphere
towards evening, when the solar rays began to decline; but it was
not properly understood until M. Prevost published his theory of the
radiation of caloric (which has since been generally adopted), which was
as follows:--“That all bodies radiate caloric constantly, whether the
objects that surround them be of the same temperature of themselves, or
not.” According to this view, the temperature of a body falls whenever
it radiates more caloric than it absorbs, and rises whenever it receives
more than it radiates; which law serves to produce an equality of
temperature. Such is exactly the case as regards the earth: during the
day it receives a supply of heat from the sun’s rays, and as it is an
excellent radiator of caloric, as soon as the shades of evening begin
to fall, the earth imparts a portion of its caloric to the air, and the
atmosphere having no means of imparting its caloric in turn, except by
contact with the earth’s surface, the stratum nearest the earth becomes
cooled, and consequently loses the property of holding so much moisture
in the state of vapour, which becomes deposited in small globular
drops. The stratum of air in immediate contact with the earth having
thus precipitated its moisture, becomes specifically lighter than that
immediately above it, which consequently rushes down and supplies its
place; and in this manner the process is carried on until some physical
cause puts a stop to it either partly or wholly. It is well known that
dew is deposited sparingly, or not at all, in cloudy weather, the clouds
preventing free radiation, which is so essential for its formation; that
good radiators, as grass, leaves of plants, and filamentous substances
in general, reduce their temperature in favourable states of the weather
to an extent of ten or fifteen degrees below the circumambient air;
and whilst these substances are completely drenched with dew, others
that are bad radiators, such as rocks, polished metal, sand, &c., are
scarcely moistened. From the above remarks it will appear evident that
dew is formed most abundantly in hot climates, and during summer in our
own, which tends to renovate the vegetable kingdom by producing all the
salubrious effects of rain without any of its injurious consequences,
when all nature seems to languish under the scorching influence of a
meridian sun.
Hoarfrost is formed when the temperature becomes so low as 32 degrees
Fahrenheit; the dew being then frozen on falling, sometimes assuming very
fantastic forms on the boughs and leaves of trees, &c., which sparkle in
the sunshine like so many gems of purest ray.