By determining the density of a gas at a given temperature and under a given pressure, we can find by the statistical method the average speed of its molecules. It depends on the most probable distribution of their energy. For hydrogen at the temperature of melting ice, and under atmospheric pressure, this speed proves to be a little over a mile a second—a speed, curiously enough, which is to that of light almost exactly as centimetres to miles. But some of the molecules are going at speeds much above the mean; fewer and fewer as the speed gets higher. Just how many there are for any assigned speed, we can calculate by the same ingenious application of unknown quantities. [Illustration: DISTRIBUTION OF MOLECULAR VELOCITIES IN A GAS.] These speeds have been found for a temperature of freezing, and as the speed varies as the square root of the absolute temperature, we might suppose that when an adventurous or lucky molecule arrived at practically the limit of the atmosphere, where the cold is intense, it would become numbly sluggish. But let us consider this. When we enclose a gas in a cooler vessel, the molecules bombard the sides more than they are bombarded back. In consequence, they lose energy; as we say, are cooled. But in free air if a molecule be fortunate enough to elude its neighbors, there is nothing to take away its motion but the ether through radiation, and this is a very slow process. Thus the escaping fugitive must arrive at the confines of the air with the speed it had at its last encounter. We reach, then, this result: In space there is no such thing as temperature; temperature being simply the aggregate effect of molecular temperament. The reason we should consider it uncommonly cold up there is that fewer molecules would strike us. Quantity, therefore, in our estimation replaces quality,—a possible substitution which also accounts for some reputations, literary or otherwise. The only forces which could affect this lonely molecule would be the heating by the Sun, the repellent force of light, and gravity. Now the speed which gravity on the Earth can control is 6.9 miles a second. It can impart this to a body falling freely to it from infinite space, and can therefore annul it on the way up, and no more. If, then, any of the molecules reach the outer boundary of the air going at more than this speed, they will pass beyond the Earth’s power to restrain. They will become little rovers in space on their own account, and dart off on interstellar travels of their own. This extension of the kinetic theory and of the consequent voyages of the molecules is due to Dr. Johnstone Stoney, who has since, humorously enough, tried to stop the very balls he set rolling. First thoughts are usually the best, after all.