68 THE ADVANCE OF SCIENCE 



is complex. In part, it contains a corol- 

 lary from the law of causation : A body 

 cannot change its state in respect of rest 

 or motion without a sufficient cause. But, 

 in part, it contains generalisations from 

 experience. One of these is that there is 

 no such sufficient cause resident in any 

 body, and that therefore it will rest, or 

 continue in motion, so long as no external 

 cause of change acts upon it. The other 

 is that the effect which the impact of a 

 body in motion produces upon the body 

 on which it impinges depends, other things 

 being alike, on the relation of a certain 

 quality of each which is called 'mass.' 

 Given a cause of motion of a certain value, 

 the amount of motion, measured by dis- 

 tance travelled in a certain time, which it 

 will produce in a given quantity of matter, 

 say a cubic inch, is not always the same, 

 but depends on what that matter is — a 

 cubic inch of iron will go faster than a 

 cubic inch of gold. Hence, it appears, 



