154 THE WOELD-ENEKGY 



to visibly affect through short distances other portions of 

 matter of like characteristics. 



And this seems the more reasonable as an explanation 

 as the force is found to be under the same law as gravi- 

 tation with regard to distance that is, the magnetic 

 attraction or repulsion is inversely as the square of the 

 distance. 



But this, it may be repeated, is in the present place 

 merely incidental. What must be held firmly in mind 

 here is, first, the action of force producing aggregations 

 of force-centers, and, secondly, the significance of such 

 aggregations. 



As each force-center is essentially an "atom," and as 

 " bodies" are built up from such "atoms," it is evident 

 that the quantity of matter within a given body will 

 depend precisely upon the number of force-centers con- 

 stituting the body. But the quantity of matter in a body 

 is called its mass. 



Evidently, then, so far as the universal law of gravita- 

 tion declares that every body attracts every other body 

 with a force whose magnitude is directly as the product of 

 their masses, it is but formulating one of the necessary 

 relations between groups of force-centers. For each 

 force-center, independently of the others in the same 

 group, must attract every force-center in the group 

 constituting the distant body, and would do so pre- 

 cisely in the same way and in the same degree were 

 it widely separated from the other members of its own 

 group. Hence, with each additional force-center in 

 any given body, such body, as a whole, must exert 

 a still greater force of attraction upon every other 

 body. 



