166 THE WOBLD-EHEKGY 



either body will be inversely as its relative mass; and,, 

 thirdly, that the velocity or ratio of approach of the two 

 bodies toward each other is a compound, constantly 

 increasing ratio which is wholly independent of mass. 



These are the fundamental phases of the motion of 

 bodies, so far as they are considered merely from the side 

 of their mutual attraction. And it is important to notice 

 that these three phases of motion are not merely three 

 different phases of relation between mass and velocity, 

 for that would be merely to substitute one term for 

 another, but also, and especially, it is important to 

 notice that they are three stages in the progressive suit- 

 ordination of mass as a factor of motion. 



And this serves to remind us again that mass and 

 velocity are reciprocal factors in the quantity of motion. 

 It is the " inert " mass that draws to itself the detached 

 and lightly moving force-sphere, though the "inert 

 mass " is itself made up of precisely similar force-spheres. 

 That which is relatively without velocity appears to 

 impart velocity, and that which is relatively without 

 mass gains increase of power through its added velocity. 



The latter statement may indeed seem, at first view, 

 to be a gratuitous one, so far as the course of our argu- 

 ment thus far can give justification. But we have only 

 to refer to the first law of motion as the law of inertia to 

 see that the justification is already implicit there. A body 

 can no more stop its own motion than it can put itself in 

 motion. And to overcome its motion requires precisely 

 the same amount of force as that expended in giving it 

 the motion it possesses. The quantity of motion a body 

 possesses is, therefore, an exact measure at once of the 

 force that has been impressed upon it in giving it motion, 



