6o LIFE AND DEATH. 



different varieties of energy. The first of these ideas 

 is older and more familiar to us, but it has not for that 

 reason a more certain existence. Energy is objective 

 reality for the same reason that matter is. The 

 latter certainly appears more tangible and more easily 

 grasped by the senses. But, upon reflection, we are 

 assured that the best proof of their existence, in both 

 cases, is given by the law of their conservation — that 

 is to say, their persistence in subsisting. 



The objective existence of matter and that of 

 energy will therefore be taken here as a postulate of 

 physical science. Metaphysicians may discuss them. 

 We have but little room for such a discussion. 



§ I. Matter. 



It is certainly difficult to give a definition of 

 matter which will satisfy both physicists and meta- 

 physicians. 



Mechanical Explanation of the Universe. Matter is 

 Mass. — Physicists have a tendency to consider all 

 natural phenomena from the point of view of mechanics. 

 They believe that there is a mechanical explanation 

 of the universe. They are always on the look out for 

 it, implicitly or explicitly. They endeavour to reduce 

 each category of physical facts to the type of the facts 

 of mechanics. They have made up their minds to see 

 nowhere anything but the play of motion and force. 

 Astronomy is celestial mechanics. Acoustics is the 

 mechanics of the vibratory movements of the air or of 

 sonorous bodies. Physical optics has become the 

 mechanics of the undulations of the ether, after having 

 been the mechanics of emission — a wonderful 



