Dew is a light, thin, and transparent vapour, slowly exhaling and ascending from the earth, in spring and summer mornings, while the sun is below the horizon, and then deposited on vegetables, in the form of small globules.
Naturalists rank dew, in general, among the numbers of watery meteors ; some, however, term it a liquefied vapour, precipitated in drops;. others, a vapour having a similar relation to frost, as rain has to snow, etc. - It is admitted, that dew cannot fall before it rises ; and that its origin and matter, no doubt, is from the vapours and exhalations of the earth and water, as will be briefly stated under the ar-ticle Evaporation.
That dews are more copious in spring than in any other season, arises from the greater stock of vapour collected on the surface of the earth, and the previous small dissipation of it during the cold and frost of the winter. Hence the truth of Pliny's remark is evident, that Egypt abounds in dews throughout the hot part of sum-rner; for, as the air during the day is too hot to condense the vapours, they never form clouds, and consequently produce no rain : thus, in climates where the days are excessively hot, and the nights remarkably cold, the vapours, rising before or after sunset, are readily converted into dew. - In the more temperate climates, they ascend and fall in greater abundance after rain than after dry weather. There are some places in which dew is observed only to rise, but never to fail; and again others, in which it is carried upward in a more consi derable proportion than downward, on account of the prevalence of winds by which it is dispersed.
Many whimsical properties and effects have, by the chemist, been attributed to common dew ; but we conceive that, in its physical nature, it differs very little from rain; except, however, that the former is more subtle or penetrating than the latter. Hence it will be found that the leather of shoes and boots is more thoroughly soaked by walking one hour in a dewy meadow, than by exposing them double that length of time to rain water. - See Leather.
It is farther remarkable, that plants continually exhale dew through the orifices of their vessels, and that this moisture is not a vapour collected by their leaves, as has often been erroneously believed. Each plant exhales this dew, according to the peculiar structure of its organs, and the situation of their orifices. Even shut up in vessels, and covered under glasses, plants have collected a greater quantity of dew during the night, than those standing in the open air. Of this nature, likewise, is the oily or honey-dew, which is sometimes exhaled by trees, as well as herbs, during the summer, and which has been found to settle on the oak, ash, etc.
May-dew, is that which falls in the beginning of summer, but especially in the month of May. It is of a yellowish colour, and many virtues are attributed to this liquid. It is principally used for whitening linen and wax ; which, if exposed to it, will gradually acquire a beautiful white.