324 POPULAR SCIENCE MONTHLY 



of it. Indeed, just where physics and chemistry leave off, I feel a real 

 and deeper problem begins. If so, the question lies at present beyond 

 the reach of natural science which biologist and physicist alike interpret 

 as the science of matter and energy. 



In what follows I shall try to review very briefly the principal ideas 

 upon which modern physics rests and shall say something about where 

 we think we have arrived in our search for knowledge. I need scarcely 

 remind you that in the natural sciences as in more practical affairs, 

 how we have arrived is as important as where we have arrived. I shall 

 therefore spend some time in presenting detached fragments of the 

 experimental evidence and inferences upon which certain conclusions 

 are based, hoping in this way to illustrate some of the constructive 

 methods of reasoning employed in research. 



The ideas which underlie all our thinking are space, time and 

 inertia or mass. With space and time as a background, the physicist 

 must pursue inertia and everything related to it, along every conceivable 

 path. In this pursuit he comes upon four ultimate though related 

 conceptions: matter, ether, electricity and energy. 



The historical development of these conceptions can not even be 

 sketched in such a " lecture as this, but it should be remembered an 

 important part of our present knowledge of matter, and nearly all that 

 we know of the ether and electricity has been gained not immediately, 

 but by inference. In so many cases we see or know directly only the 

 first and last link of a chain of events and must search by indirect 

 means for the mechanism lying between. 



At bottom, I suppose, the ether, electricity, force, energy, molecule, 

 atom, electron, are but the symbols of our groping thoughts, created by 

 an inborn necessity of the human mind which strives to make all things 

 reasonable. In thus reasoning from things seen and tangible, to things 

 unseen and intangible, the resources of mathematical analysis are ap- 

 plied to the mental images of the investigator, images often suggested 

 to him by his knowledge of the behavior of material bodies. This 

 process leads first to a working hypothesis, which is then tested in all 

 its conceivable consequences, and any phenomena not already known 

 which it requires for its fulfilment, are sought in the laboratory. By 

 this slow advance a working hypothesis which has satisfied all the de- 

 mands put upon it gradually becomes a theory which steadily gains in 

 authority as more and more new lines of evidence converge upon it and 

 confirm it. 



If we now consider more closely the nature of the conceptions, 

 matter, ether, electricity and energy, we shall later find that matter, 

 ether and electricity possess some attributes in common, and if we take 

 careful heed to what we shall understand by the word, we may call them 

 substances. Energy appears as the measure of their possible inter- 

 actions. 



