Colourless glass is very readily suffused with dampness, but polished metals are not so, even when dews are heavily condensed on other bodies. To comprehend fully the phenomena of the formation of dew, we must remember that the entire surface of the earth is constantly radiating heat into space; and that, as by night no absorption is taking place, it naturally cools.[55] As the substances spread over the earth become colder than the air, they acquire the power of condensing the vapour with which the atmosphere is always charged. The bodies which cover this globe are very differently constituted; they possess dissimilar radiating powers, and consequently present, when examined by delicate thermometers, varying degrees of temperature. By the researches of Dr. Wells,[56] which may be adduced as an example of the best class of inductive experiments, we learn that the following differences in sensible heat were observed at seven o’clock in the evening:-- The air four feet above the grass 60-3/4 Wool on a raised board 54-1/2 Swandown on ditto 53 The surface of the raised board 57 Grass plat 51 Dew is most abundantly deposited on clear, calm nights, during which the radiation from the surface of the earth is uninterrupted. The increased cold of such nights over those obscured by clouds is well known. The clouds, it has been proved, act in the same way as the screens used by gardeners to protect their young plants from the frosts of the early spring, which obstruct the radiation, and, in all probability, reflect a small quantity of heat back to the earth. It is not improbable that the observed increase in grass crops, when they have been strewn with branches of trees or any slight shades, may be due to a similar cause.[57]