The beautiful phenomenon of the formation of dew is also distinctly connected with the peculiar properties which we have been studying. When from the bright blue vault of heaven, the sparkling constellations shower their mild light over the earth, the flowers of the garden and the leaves of the forest become moist with a fluid of the most translucid nature. Well might the ancients imagine that the dews were actually shed from the stars; and the alchemists and physicians of the middle ages conceive that this pure distillation of the night possessed subtile and penetrating powers beyond most other things; and the ladies of those olden times endeavour to preserve their charms in the perfection of their youthful beauty through the influences of washes procured from so pure a source.[53] Science has removed the veil of mystery with which superstition had invested the formation of dew; and, in showing to us that it is a condensation of vapour upon bodies according to a fixed law of radiation, it has also developed so many remarkable facts connected with the characters of material creations, that a much higher order of poetry is opened to the mind than that which, though beautiful, sprang merely from the imagination. Upon the radiation of heat depends the formation of dew, and bodies must become colder than the atmosphere before it will be deposited upon them. At whatever temperature the air may be, it is charged to saturation with watery vapour, the quantity varying uniformly with the temperature. Supposing the temperature of the air to be 70° F., and that a bottle of water at 60° is placed in it, the air around the bottle will be cooled, and will deposit on the glass exactly that quantity of moisture which is due to the difference between the temperature of the two bodies. Different substances, independent of colour, have the property of parting with heat from their surfaces at different rates. Rough and porous surfaces radiate heat more rapidly than smooth ones, and are consequently reduced in temperature; and, if exposed, are covered with dew sooner than such as are smooth and dense. The grass parterre glistens with dew, whilst the hard and stony walk is unmoistened.[54]