66 THE ADVANCE OF SCIENCE 



line for any conceivable time or distance, 

 unless it is interfered with ; that any 

 change of motion is proportional to the 

 'force' which causes it, and takes place 

 in the direction in which that 'force' is 

 exerted ; and that, when a body in mo- 

 tion acts as a cause of motion on another, 

 the latter gains as much as the former 

 loses, and vice versa. It is to be noted, 

 however, that while, in contradistinction 

 to the ancient idea of the inherent tend- 

 ency to motion of bodies, the absence of 

 any such spontaneous power of motion 

 was accepted as a physical axiom by the 

 moderns, the old conception virtually 

 maintained itself in a new shape. For, 

 in spite of Newton's well-known warning 

 against the ' absurdity ' of supposing that 

 one body can act on another at a distance 

 through a vacuum, the ultimate particles 

 of matter were generally assumed to be 

 the seats of perennial causes of motion 

 termed 'attractive and repulsive forces,' 



