The relations would be quite different if the earth had the 11.6 times greater diameter of Jupiter, or the diameter of Saturn, which is 9.3 times greater than its own. The radiation from the earth to the moon would then make up about a sixth or a ninth of the actual solar radiation, taking the temperature of the earth’s surface at 360°. We can easily calculate, further, that Jupiter and Saturn would radiate as much heat against a moon at a distance of 240,000 or 191,000 km. respectively (since the distance of the moon from the earth amounts to 384,000 km.) as the sun sends to Mars—taking the temperature of those planets at 360° Cent. Now we find, near Jupiter as well as near Saturn, moons at the distances of 126,000 and 186,000 km. respectively, which are smaller than those mentioned, and it is not inconceivable that these moons receive from their central bodies sufficient heat to render life possible, provided that they be enveloped by a heat-absorbing atmosphere. The conditions appear to be less favorable for the innermost satellites of Jupiter and Saturn. When their planets are shining at the maximum brilliancy, their light intensity is only a sixth or a ninth of the solar light intensity, which upon these satellites is itself only one-twenty-seventh or one-ninetieth of the intensity on the earth. During the incandescence epoch of these planets their moons will certainly for some time have been suitable for the development of life. That the atmospheric envelopes limit the heat losses from the planets had been suggested about 1800 by the great French physicist Fourier. His ideas were further developed afterwards by Pouillet and Tyndall. Their theory has been styled the hot-house theory, because they thought that the atmosphere acted after the manner of the glass panes of hot-houses. Glass possesses the property of being transparent to heat rays of small wave lengths belonging to the visible spectrum; but it is not transparent to dark heat rays, such, for instance, as are sent out by a heated furnace or by a hot lump of earth. The heat rays of the sun now are to a large extent of the visible, bright kind. They penetrate through the glass of the hot-house and heat the earth under the glass. The radiation from the earth, on the other hand, is dark and cannot pass back through the glass, which thus stops any losses of heat, just as an overcoat protects the body against too strong a loss of heat by radiation. Langley made an experiment with a box, which he packed with cotton-wool to reduce loss by radiation, and which he provided, on the side turned towards the sun, with a double glass pane. He observed that the temperature rose to 113° (235° F.), while the thermometer only marked 14° or 15° (57° or 59° F.) in the shade. This experiment was conducted on Pike’s Peak, in Colorado, at an altitude of 4200 m. (13,800 ft.), on September 9, 1881, at 1 hr. 4 min. P.M., and therefore at a particularly intense solar radiation.