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smaller particles can float in thinner air. We see the principle exemplified in our terrestrial clouds; the low nimbus being formed of comparatively large globules, while the high cirrus is made up of very minute particles. If we go yet higher, we reach a region incapable of supporting clouds of any kind, so rarefied is its air. This occurs about five miles above the earth's surface; and yet even at this height the density of our air is greater than is the probable density of the air at the surface of Mars. We see, therefore, that the Martian atmosphere should from its rarity prove cloudless, just as we observe it to be. So far in this our investigation of the Martian atmosphere we have been indebted solely to the principles of mathematics and molar physics for help, and these have told us something about the probable quantity of that atmosphere, though silent as to its possible quality. On this latter point, however, molecular physics turns out to have something to say; for an Irish gentleman, Dr. G. Johnstone Stoney, has recently made an ingenious deduction from the kinetic theory of gases bearing upon the atmosphere envelope which any planet can retain. His deduction is as acute as it appears from observation to be in keeping with the facts. It is this:-- The molecular theory of gases supposes them to be made up of myriads of molecules in incessant motion. What a molecule may be nobody knows; some scientists supposing it to be a vortex ring in miniature,--something like the swirl produced by a teaspoon when drawn through a cup of tea. But whatever it be, the idea of it accounts for the facts. The motion of the molecules is almost inconceivably swift as they dart hither and thither throughout the space occupied by the gas, and their speed differs for different gases. It is calculated that the molecules of oxygen travel, on the average, at the rate of fifteen miles a minute, those of water vapor about twenty miles a minute, and those of hydrogen, which are the fastest known, at the enormous speed of a mile a second. But this average velocity may, in any particular case, be increased by collisions of the molecules among themselves something like sevenfold. What is more, each molecule of the gas is bound, sooner or later, to attain this maximum velocity of its kind merely on the doctrine of chances. When it is attained, the molecule of oxygen travels at the rate of one and three c a ” tat 1 1 c ’ co 1 ee rT fourths miles a second, the molecule of water vapor at the rate of two and one third miles a second, and the molecule of hydrogen actually at seven miles a second, six hundred times as fast as our fastest express train. Now, if a body, whether it be a molecule or a cannon-ball, be projected away from the earth's surface, the earth will at once try to pull it down again: this instinctive holding on of Mother Earth to what she has we call gravity. In the cases with which we are personally familiar, her endeavor is eminently successful; what goes up usually coming down again, either on the thrower or on some other person. But even the earth is not omnipotent. As the velocity with which the body is projected increases, it takes the earth longer and longer to overcome it and compel the body's return. Finally there comes a speed which the earth is just able to overcome, if she take an infinite time about it. In that case, the body would continue to travel away from her, at a constantly diminishing rate, but still at some rate, on and on into the depths of space, till it attained infinity, at which point the truant would stop, and reluctantly begin to return again. This velocity we may call the critical velocity. It is the velocity which the earth would cause in a body falling to