43 2 SCIENCE PROGRESS. 



which appears as gravitation ; and finally chemical proper- 

 ties, i.e., chemical energy. It is always a question of 

 energy alone, and if we imagine the various kinds of energy 

 removed from matter there remains nothing, not even the 

 space it occupied ; for space makes itself known only 

 through the expenditure of energy which the penetration 

 into it requires. Matter is therefore nothing but a group 

 of various forms of energy co-ordinated in space, and all 

 that we try to say of matter is really said of these energies. 



What I am endeavouring to lay down is so important 

 that you will pardon my venturing to approach the subject 

 from another quarter. Allow me to use for this purpose 

 the most drastic illustration I can find. Imagine that you 

 receive a blow from a stick. What do you feel, the stick 

 or its energy ? 



The only possible answer is : The energy. For the 

 stick is the most harmless thing in the world as long as 

 it is not wielded. But we can also knock ourselves 

 against a stationary stick. Certainly. What we perceive, 

 as already stated, are differences of energy conditions 

 relative to our sense organs, and it is consequently im- 

 material whether the stick moves towards us or we towards 

 the stick. If both we and the stick are moving in the 

 same direction with the same velocity, the latter has no 

 further existence for our sense of feeling, for it can no 

 longer come in contact with us and effect an exchange of 

 energy. 



These considerations show, I hope, that all that we 

 have until now been able to express by the ideas of 

 Matter and Force — and indeed much more besides — may 

 actually be expressed by the idea of energy. It is a ques- 

 tion simply of transferring to this conception those proper- 

 ties and laws which were formerly ascribed to matter and 

 force. We gain further the enormous advantage of doing 

 away with the contradictions which were attendant on the 

 former method of treatment, and to which I alluded in the 

 earlier part of my paper. By making no assumption as to 

 the relation between the different forms of energy, except 

 that given by the law of conservation, we leave ourselves 



