598 POPULAR SCIENCE MONTHLY. 



This decisive side of the new theory will come more plainly 

 into view if I present the concept in a brief historical sketch. 

 We have already seen that the progress of science is marked by 

 the discovery of more and more general invariants ; and I have 

 shown how the first of these unchangeable entities, mass, has ex- 

 panded into matter that is, mass endowed with volume, weight, 

 and chemical properties. Yet this idea was obviously insuffi- 

 cient from the beginning to cover the incessant variability of 

 phenomena ; and since Galileo's time, that of force has been added 

 as a means of accounting for it. Yet force was destitute of the 

 property of unchangeableness ; and after functions were found in 

 mechanics, in living force, and work which manifested themselves 

 as partial invariants, Mayer discovered in energy the most uni- 

 versal invariant, dominating over the whole domain of the physi- 

 cal forces. 



Conformably to this historical development, matter and energy 

 continued existing together, and all that we could say of their 

 mutual relations was that they mostly appeared with one another, 

 or that matter was the bearer or the vessel of energy. 



Are matter and energy, then, really different from one another 

 like, for instance, body and soul ? Or is not rather what we know 

 and express of matter already comprised in the idea of energy, so 

 that we can represent all phenomena with this one entity ? In 

 my opinion the answer can not be doubtful. What is included in 

 the idea of matter is, first, mass that is, the capacity for momen- 

 tum ; next, volume, or space-filling energy ; then gravity, or the 

 peculiar kind of static energy which is manifested in general 

 weight; and, lastly, chemical properties, or chemical energy. 

 We still have only energy ; and if we think on about these vari- 

 ous kinds of matter, there is nothing else left us, not even the 

 space it occupies, for this, too, is knowable only by the application 

 of energy, which is required to compress matter within it. Hence 

 matter is nothing but a spatially contiguous group of different 

 eaergies, and all that we can predicate of it we predicate only of 

 these energies. 



What I am at pains to assert here is so important that you 

 will pardon me if I try to approach the subject more closely from 

 another side. Permit me, therefore, to take the most drastic ex- 

 ample I can find. Imagine that you are struck with a stick. 

 What do you feel, the stick or its energy ? The only possible an- 

 swer is the energy, for the stick is the most harmless thing in the 

 world, so long as it is not made to strike. But we can also run 

 against a stick at rest. Very true ; and what we feel then is, as I 

 have already declared, the difference in the conditions of energy 

 as opposed to our sense organs ; and it is therefore all the same 

 whether the stick is struck against us or we are pushed against 



