48 THE POPULAR SCIENCE MONTHLY. 



their orbits have slowly undergone slight changes. But the simple 

 law of universal gravitation, combined, of course, with the laws of 

 motion, not only leads to Kepler's laws as a very close approximation 

 to the actual motions, but also accounts for those slight changes which 

 have just been mentioned as necessary to make Kepler's laws fit obser- 

 vation exactly. We are inevitably led to regard the attraction of 

 gravitation as the cause which keeps the planets in their orbits. 



But it may be said, What is the difference in the two cases ? Is not 

 the law of gravitation merely a simpler mode of expressing the observed 

 facts of the planetary motions just like the somewhat less simple laws 

 of Kepler ? What right have we to introduce the idea of causation in 

 the one case more than in the other ? 



The answer to this appears to be that in the one case, that of 

 Kepler's laws, supposing them to be true, we have merely a statement 

 of what, on that supposition, would be a fact regarding the motions of 

 the planets, whereas in the other case the observed motions are referred 

 to a pi'operty of matter of the operation of which in other and perfect- 

 ly different jihenomena we have independent evidence. 



I have purposely omitted to mention the important difference be- 

 tween the two cases, which lies in the circumstance that Kepler's laws 

 require correction to make them applicable to long intervals of time, 

 whereas the law of gravitation shows no sign of failure ; because, even 

 if the former had been perfectly exact, however long the interval of 

 time to which they were applied, I doubt if they would have carried 

 with them the idea of causation. 



To take another simple illustration, let us think of the propulsion 

 of a bullet in an air-gun. We speak of the motion of the bullet as 

 being caused by the elasticity of the compressed air. And the idea of 

 causation comes in because we refer this particular instance of motion 

 to a property of gas, of the existence and operation of which we have 

 evidence in perfectly independent phenomena. 



It is thus that in scientific investigation we endeavor to ascend from 

 observed phenomena to their proximate causes ; btit, when we have 

 arrived at these, the question presents itself. Can we in a similar man- 

 ner regard these causes in turn as themselves the consequences of some 

 cause stretching still further back in the chain of causation ? If the 

 motion of the bullet in an air-gun be caused by the elasticity of the 

 compressed air, can we account for the elasticity of a gas ? If the re- 

 tention of the planets in their orbits be due to the attraction of gravi- 

 tation, can we explain how it is that two material bodies should attract 

 one another across the intervening space ? 



Till a time M^ell on in the present century, we could only take the 

 elasticity of gases as a fact, and deduce the consequences which flow 

 from it. But the researches of Joule and Clausius and Maxwell and 

 Crookes and others have accumulated so much evidence in favor of 

 the general truth of the kinetic theory of gases, that we are now dis- 



