Exhalation is distinguished from evaporation by some writers, as not applying to the raising of vapour in the ordinary sense of the word, but to subtle, dry effluvia, loosened, by the action of the sun, from minerals and other hard terrestrial bodies; that these exhalations ascend until their specific gravity equals that of the surrounding atmosphere, where, mixing with other vapours, they help to form clouds, and return to the earth in mists, dews, rains, etc..