THE PHILOSOPHICAL BASIS 25 



work or energy as a means of co-ordinating the 

 phenomena, instead of stating them in terms 

 of force as Newton did. Although it gave a 

 more intimate insight into mechanical processes, 

 Newton's method was perhaps less general than 

 that of Huygens, which often enables us to pass 

 directly from a knowledge of the initial to a 

 prediction of the final state of a system, and to 

 avoid the difficulties of tracing its intermediate 

 operations. In the history of mechanical science, 

 now one method and now the other has proved 

 the more useful; and, in the wider field of physics, 

 the two schools are still represented, on the one 

 hand, by those who seek to trace the intimate 

 processes of change by means of molecular 

 theories, and, on the other, by those who rely 

 on a more general presentment, which avoids 

 such hypotheses by the use of the principles of 

 thermodynamics. 



By simple experiments, such as those described 

 above, the relative masses of two reacting bodies 

 may be measured by the constant inverse ratio of 

 their accelerations. It follows that the product of 

 the mass and the acceleration is the same for the 

 two bodies. Thus the force which the first body 

 exerts on the second is the same as the force 

 which the second exerts on the first ; or, as 

 Newton expressed it, action and reaction are 

 equal and opposite. 



The conception of mass, in the present sense 

 of the word, we owe to Newton : before his day 

 no clear distinction was made between mass and 

 weight. On the principle of relativity mass and 

 weight are necessarily connected, but, as defined 

 above, we cannot predict whether mass has any 



