"Whether or not that be the chief adjustment by which the right temperature is secured, it is at least a very important item. The intensity of the sun's heat must, of course, be considered in connection with its distance from the earth. The distance of the sun is no less important than the power of his rays; indeed, in one sense, it is more important, for if the intensity of the sun's heat were doubled, the temperature of the earth would be increased only twofold; whereas, if the earth were brought to one-half its present distance from the sun, the heat would be increased four times. Heat being one of the radiant forces, its intensity diminishes in proportion to the square of the distance through which it acts. If the earth were 190,000,000 of miles from the sun instead of 95,000,000, as it now is, the force of the sun's rays would be diminished fourfold. The Creator has so fixed the distance of the earth and sun, and the power of the sun's heat, as to give to this world a temperature suited to its various inhabitants. "The temperature of the earth has also some dependence upon our atmosphere. Can you tell us, Ansel, how the temperature of the earth is affected by the atmosphere?" "You have already told us that the atmosphere is _diathermic_, allowing the heat of the sun to fall upon the earth almost undiminished in force. If the air were so constituted as to intercept the sun's rays, it is plain that the earth would receive less heat." "This adaptation of our atmosphere to transmit the sun's rays," said Mr. Wilton, "is more subtle than it appears at first sight. It is not merely a matter of depth and density, though those are important considerations, nor is it merely a question of the elements of which the atmosphere is composed. Simple gases are _diathermic_. The atmosphere is therefore made up of two simple gases, oxygen and nitrogen, not chemically combined, but mixed together. Compound gases intercept the passage of heat. Ammonia, composed of hydrogen and nitrogen chemically united, almost wholly stops it. Even ozone, which is nothing but oxygen in a changed or _allotropic_ state, is not _diathermic_. The _diathermic_ quality of the air depends, then, not only upon the fact that it is composed of simple elements mingled, but not chemically joined, but also upon the _state_, or _condition_, of those simple elements.