280 THE KINETIC THEORY OF GASES. 



planet at a rate which would carry it away from the planet more rapidly 

 than the planet can draw it back, that molecule will escape into space. 

 It is not theoretically impossible, although practically unrealizable, to 

 construct a gun which would fire a bullet vertically into the air at such 

 a rate that the bullet might never return to the earth. What, then, 

 would occur to it? Well, it would wander on through space as a little 

 planet, performing an ellipse round the sun, as, indeed, many aerolites 

 or "shooting stars" are known to do. It might, indeed, chance to 

 come within' the range of attraction of some planet — e. g., Jupiter — 

 massive enough to hold it; or it might actually fall on the surface of a 

 planet; in the former case, it would act like a little satellite, and revolve 

 round that planet, as the numerous stones of which Saturn's rings are 

 composed revolve round Saturn ; in the latter case, it would simply 

 become part of that planet, as the falling stars which reach the earth 

 form, after their fall, a portion of the earth. 



The molecule of gas, which we have been considering, differs in no 

 particular from a bullet in its wanderings or in its fate. If it chance 

 to come within the sphere of attraction of a planet of sufficient mass 

 to retain it, it will, according to Dr. Stoney, form part of that planet's 

 atmosphere. If not, it will wander on, until it may, by chance, come 

 near enough to the sun to fall a victim to its enormous attractive force, 

 and it will then become part of the sun's atmosphere. 



Dr. Stoney has summed up the results of various inquiries of this 

 kind in a memoir entitled Of Atmosphere upon Planets and Satellites. ' 



One important point has been omitted in the sketch given of the 

 kinetic theory. It is this : When it was said that a molecule of oxygen 

 moves at the rate of about one third of a mile per second, it was not 

 implied that all molecules are moving at that. rate. Some, urged on 

 by collisions from behind, acquire* a much more rapid rate; others, 

 hindered in their motion by collisions with other molecules moving 

 more slowly than themselves, or iu an opposite direction, have their 

 rate of motion decreased. A gas must be conceived as composed of 

 an almost infinite number of such molecules, jostling each other in 

 every conceivable way. The rate of one-third of a mile per second, 

 deduced by Clausius as the average rate of motion of a molecule of 

 oxygen, must be understood to mean that if all the rates of motion 

 were to be balanced out, so that the swiftly moving molecules gave up 

 some of their motion to the slowly moving molecules, and vice versa, 

 the molecules would all be moving at the above mentioned rate. But 

 it must be distinctly borne in mind that this imaginary state of things 

 never occurs. There are always many molecules moving faster, many 

 slower, than the average. 



I find in my own case that it helps greatly to a clear understanding of 

 such a conception as that of which a short account has been given if 

 a mental picture can be called up which will illustrate the conception, 



"'Royal Dublin Society," Vol. VI, November, 1897, pp. 305-328. 



