8 Mr. J. Croll on What determines Molecular Motion ? — 



ertion of force. Hence, be the cause of the determination what- 

 ever it may, it cannot possibly be an act or exertion of force. 



In a similar manner we can prove that motion cannot be de- 

 termined by motion. Motion will produce motion, but motion 

 cannot determine motion. A ball A in motion will produce 

 motion in a ball B, but the motion of the ball A will not deter- 

 mine the motion of the ball B, either in regard to direction or to 

 the times of its happening. The particular direction taken by 

 the ball B is not due to the motion of A, but to the particular 

 direction in which A is moving at the moment in which it pro- 

 duced motion in B ; so that the direction taken by B must be 

 referred not to the motion of A, but to that something, whatever 

 it may be, which causes A to move in the particular direction in 

 which it moves. In other words, the determinate direction 

 taken by B is not due to the motion of A, but to the direction of 

 the motion of A. In like manner it can be proved that the di- 

 rection taken by A is not due to the motion of some other body 

 (say C), but to the direction of that moving body C. 



In a similar way we can prove that the particular time at which 

 B begins to move is not due to the motion of the striking body 

 A^ but to the particular time at which the body A strikes B. 



As attraction and cohesion play important parts in all physical 

 phenomena, it will be as well to satisfy ourselves that the deter- 

 mination of motion cannot be referred to these. 



Suppose two particles, A and B, are at some distance from 

 each other, A being to the east of B. Let them move toward 

 each other under the influence of their mutual attraction, A to 

 the west and B to the east. Their motions are due to attraction, 

 but not the direction of their motions. Attraction is the reason 

 why A moves, but not the reason why it moves west rather than 

 east. This is not due to attraction, but to the fact that B hap- 

 pened to be on the west side of A when the motion took place. So 

 the direction taken must not be referred to attraction, but to 

 that something, whatever it may have been, which was the cause 

 why B happened to be on the west side of A. 



In regard to attraction under every form, the direction taken 

 by moving particles is due to the prearrangement of those par- 

 ticles in regard to time and space. A difference in prearrange- 

 ment would necessarily produce a corresponding difference in the 

 directions taken by the particles. 



Or let us take another very simple case of motion, viz. the 

 motion of a planet in a circular orbit. Most people at first 

 would be inclined to refer the circular path taken by the planet 

 to the action of forces. They would say that the form of the 

 orbit is produced by the action of the two forces, centrifugal and 

 centripetal. But if we examine the matter properly, we shall 



