=At the Height of Fifty Miles.= There is enough air here to refract light slightly, as at twilight, and to render luminous the meteors that rush with fearful velocity against its widely scattered molecules. At this distance from the earth there probably is no more air than would be found under the receiver of the best air pump, and, the reader will be surprised to learn, darkness is practically complete, although the hour may be midday, for there are no dust motes to scatter and diffuse and render visible the light rays of the sun. (See Chapter III.) =The Darkness of Outer Space.= It may be proven by taking an inclosed volume of air, freeing it of dust motes, of which there are millions per cubic centimeter, and then trying to illuminate it; it will be found that no matter how powerful the light directed into it, it remains wholly dark. When one looks upward on a clear day, he apparently sees the whole universe illuminated; but in point of fact only the thin stratum of the earth’s air in which he lives is illuminated. Outer space is practically without temperature or light. The rays of the sun do not become either light or heat or electricity until they encounter the molecules of the air, or the invisible dust motes, or the cloud particles near the earth and through interference are transmuted from etheric vibrations into other forms of energy. =The Bacteria of Disease and of Putrefaction.= These rapidly diminish in number with elevation, and on the tops of the highest mountain peaks practically none are found. Mid-ocean also shows but few. =At the Height of Twenty-five Miles.= Air, light as it is, has still sufficient density to obstruct the passage of the minutest wave lengths of light, and here probably begins to be appreciable the blue tint of the heavenly vault. At this short distance from the earth there must be a deathlike stillness, for there is no medium sufficiently dense to transmit sound. Two persons could not hear each other speak, even if they could live in this rare atmosphere, which they could not. Here is eternal peace and no apparent motion, for storms and ascending and descending currents cease long before this level is reached. The cold is intense and daylight but a feeble illumination. There are no clouds. =Isothermal Stratum Entered at the Height of Seven Miles.= We know that the temperature decreases rapidly with ascent—about one degree for each three hundred feet—until the top of the storm level is reached, at about seven miles, when a most wonderful discovery is made: the thermometer no longer falls as the aviator rises, or as balloons float to great altitudes carrying self-registering instruments. The temperature remains practically stationary, so far as exploration has been made, which is to the height of over nineteen miles. Major R. W. Schroeder, U. S. A., flew in an aëroplane to 36,000 feet and recorded a temperature of 69° below zero.