Dbcember 2, 1898.] 



SCIENCE. 



Ill 



slowly than themselves, or in opposite di- 

 rectionSjhave 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 un- 

 derstood 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 mole- 

 cules, 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 concep- 

 tion, although even imperfectly. Some such 

 picture may be formed by thinking of the 

 motions of the players in a game of foot- 

 ball. At a critical point in the game the 

 players are running, some this way, some 

 that; one has picked up the ball and is 

 running with it, followed by two or three 

 others ; while players from the opposite 

 side are slanting towards him, intent upon 

 a collision. The backs are at rest, per- 

 haps ; but on the approach of the ball to 

 the goal they quicken into activity, and 

 the throng of human molecules is turned 

 and pursues an opposite course. The fail- 

 ure of this analogy to represent what is 

 believed to occur in a gas is that the 

 players' motion is directed and has a pur- 

 pose ; that they do not move in straight 

 lines, but in any curves which may suit 

 their purpose ; and that they do not, as 

 two billiard balls do, communicate their 

 rates of motion one to the other by colli- 

 sion. But, making such reservations, some 



idea may be gained of the encounters of 

 molecules by encounters in a football field. 



In considering averages it is clear that 

 there must be a practical limit on both sides 

 of the mean. If a man throws dice he may 

 turn up sixes thrice in succession, or some 

 greater number of times, by chance ; but it 

 is clear he will not go on throwing sixes for 

 ever, though there is no absolute reason why 

 he should not. Similarly, in thinking of 

 the rates of motion of molecules there will 

 be a pi-actical limit of rate at which any one 

 molecule will move. It is unlikely that any 

 one molecule will cease to move for any ap- 

 preciable time ; and it is unlikely, too, that 

 any one molecule will develop any excep- 

 tionally rapid velocity, say twenty times the 

 mean. Still, such events may conceivably 

 occur; they will,however,be very infrequent. 



Those gases which are light, and whose 

 molecules have a high intrinsic average rate 

 of motion, will, in the nature of things, con- 

 tain some molecules which happen to be 

 moving at a high speed, and necessarily 

 will contain more such than a gas of higher 

 density, the average rate of motion of whose 

 molecules is slower. It may happen that 

 molecules of each kind, of gas with low as 

 well as of gas of high density, may possess 

 such exceptionally high velocity at the con- 

 fines of our atmosphere, where there are 

 comparatively few gaseous molecules alto- 

 gether ; and it may also happen that these 

 molecules are moving in a direction more or 

 less nearly perpendicular to the surface of 

 the planet, and it may also happen that 

 such molecules suffer no collisions in their 

 vertical path ; if these events all happen 

 the molecules will escape. But as, on the 

 doctrine of chances, there are more mole- 

 cules of light gas endowed with such ex- 

 ceptionally high velocity than there are of 

 heavy gas, more molecules of the former 

 will escape away from the neighborhood of 

 the planet, and enter free space as inde- 

 pendent entities, than of the latter. 



