Experience has proved that the conditions of the sun’s rays are not always the same; and there are few persons who have not observed that a more than usual scorching influence prevails under some atmospheric circumstances. This is also evidenced in the effects produced on the foliage of trees, which, though often attributed to electricity, is evidently due to heat. An examination of the solar radiations, as exhibited in the prismatic spectrum, has proved the existence of a class of heat rays, which manifest themselves by a very peculiar deoxidizing power quite independent of their caloric properties, to which the name of _parathermic rays_ has been given.[48] We are protected from the severe effects of these rays by the ordinary state of the medium through which the solar heat passes. Our atmosphere is a mixture of gases and aqueous vapour; and it has been found, as already stated, that even a thin film of water, however transparent, prevents the passage of many calorific radiations, and the rays retarded are, for the most part, of that class which have this peculiar scorching power. The air is, in this way, the great equaliser of the solar heat, rendering the earth agreeable to all animals, who, but for this peculiar absorbent medium, would have to endure, even in our temperate clime, the burning rays of a more than African sun. The surface of the earth during the sunshine--and, though in a less degree, even when the sun is obscured by clouds--is constantly receiving heat; but the rate of its absorption varies. Benjamin Franklin showed, by a set of simple but most conclusive experiments, that a piece of black cloth was warmed much sooner than cloth of a lighter colour;[49] and we know, from observations of a similar class, that the bare brown soil receives heat more readily than the bright green grassy carpet of the earth. Consequently, during the winter season, relatively to the quantity poured from its source, more heat penetrates the uncovered soil, than during the spring or summer. There is a constant tendency to an equilibrium; and, during the night, the surface is robbed of more heat, by the colder air, than by day; as, when the earth is not receiving heat, it is constantly radiating it back into space. Even in these processes of convection and radiation, a similar law prevails to that which is discovered in examining into the rate of calorific absorption.