282 THE KINETIC THEORY OF GASES. 



The next question to which Dr. Stoney addresses himself is : What 

 rate of motion must a molecule have in order that it may escape from 

 the attraction of the earth ? The least velocity which will enable such 

 a molecule to escape is about 7 miles per second. And it is assumed, 

 from observations taken at high altitudes, that the temperature of the 

 upper regions of the atmosphere is about —06° C, or about —87° F. 



This velocity of 7 miles a second is, however, considerably greater 

 than the average velocity of a molecule of hydrogen, which, at 32° F., 

 it will be remembered, is only about a mile and a quarter. But it is 

 not greater than the velocity of some of the molecules; and these will 

 therefore escape. In fact, Dr. Stoney concludes that in every gas a 

 considerable proportion of the molecules have a velocity at least ten 

 times as great as the mean. 



Now on this earth the important constituents of the atmosphere are 

 nitrogen, oxygen, argon, carbon dioxide, water vapor, and ammonia; 

 and their densities are as follows, that of hydrogen being taken as 

 unity : 



Nitrogen 14 



Oxygen 16 



Argon 20 



Carbon dioxide 22 



Water gas . 9 



Ammonia 8.5 



We are here chiefly concerned with the gases of the earth's atmos- 

 phere; but it may be of interest to cast a glance at other conclusions 

 which follow from Dr. Stoney's speculations. 



The moon, che mass of which is much less than that of the earth, 

 would retain a gas of density 40, or thereabouts; but all less dense 

 gases would escape rapidly. From the planet Mercury water vapor 

 would at once escape, and it is probable that both nitrogen and oxygen 

 would escape more slowly. Argon and carbon dioxide might, however, 

 be permanent constituents of the atmosphere of Mercury. Venus, on 

 the other hand, retains water vapor; but lighter gases would escape. 

 It must be remembered that if the water were to escape from a planet 

 in the state of vapor, its place would be at once supplied by evapora- 

 tion of planetary seas, if there were any, and that, in the long run, all 

 the water would, in the state of gas or water, leave the planet. 



Indeed, Dr. Stoney thinks it not unlikely that we are slowly losing 

 our stock of water. This, however, need excite no alarm, and our 

 water will probably outlast our coal many millions of years. For so 

 few of the molecules of water comply with the required standard of 

 velocity that the rate of loss is almost infinitesimally small. 



Similarly Dr. Stoney conjectures that water can not remain on Mars;* 

 that all known gases would be imprisoned by Jupiter; and that Saturn, 

 Uranus, and Neptune may probably be able to retain all gases heavier 

 than hydrogen. As for the sun, its mass is so enormous relatively to 



I 



