But it is not certain in every case that all such planets possess an atmosphere. Observations of the Moon when passing some star show that the air envelope, if present, is unable to deflect the light beam from the star, or in other words it has no perceptible power of refraction. From this we also infer that its density is very small, corresponding at most to one or two mm. (.04 to .08 inches) barometric pressure. But we have good reasons to believe that the Moon has been detached from the Earth, carrying away parts of its lightest substance, which theory is supported by the fact that the Moon’s mean density (3.3) is only six-tenths of the Earth’s (which again is 5.53 times that of water), and we might therefore have expected that the Moon in parting should have shared in the very lightest constituents of the Earth, namely its air-covering. Such was unquestionably the actual procedure, but in the course of time the Moon has lost its originally no doubt considerable atmosphere. The reason is that the molecules in a gas are in a continuous rapid motion, which is the swifter the lighter the gas and the higher its temperature. In hydrogen, the lightest gas known, the velocity amounts to 1.84 km. (1.15 miles) per second at 0° C. (32° F.). The parts of the Moon exposed to the strongest sunlight are heated to about 150° C. (300° F.). At that temperature, the average velocity of hydrogen molecules is 2.29 km. (1.43 miles) per second. But a body departing from the surface of the Moon at a rate of 2 km. (1.24 miles) per second, or more, cannot be retained by the attraction of that globe and therefore never retraces its path but speeds away for ever. In the same manner a bullet ejected from a cannon with an initial velocity of 11.2 km. (7 miles)--a velocity not even approached by present artillery--would fly away from Earth barring the resistance of the air. Thus we see that we as yet are far from the realization of the dreams of Jules Verne in his _A Voyage to the Moon_. At any rate, gravity on the Moon is too weak to retain hydrogen over the hottest point of the surface. This part of the gas flies away, new supplies rush in from the sides, and in a short time all traces of hydrogen have disappeared from the Moon. Probably it was mainly gathered in by the Sun, where a velocity of 613 km. (380 miles) per second is necessary if the molecules are to overcome the Sun’s attraction, while their actual velocity there amounts to only about 8 km. (5 miles) per second.