APPENDIX 355 



nothing, we have the intuition of energy ; but when 

 we tread on the steps, and so raise our body, we have the 

 intuition of force. Force is that which accelerates the 

 velocity of a mass. If the latter is at rest, we consider 

 it to have zero velocity. If it is moving, and we stop 

 it, there is still acceleration, but this is negative. 



Matter, that is, the substantia physica, is clearly to 

 be conceived only in terms of energy. It is, to our 

 direct intuitions, resistance, or inertia, that which re- 

 quires energy in order that it may be made to undergo 

 change. Our static idea of physical solidity, or 

 massiveness, disappears on ultimate analysis. Mole- 

 cules are made up of atoms, and the atoms are assumed 

 to have all the characters of matter : we could not see 

 them, of course, even if we possessed all the magnifying 

 power that we wished, for they would be too small to 

 reflect light. Modern physical theory is compelled to 

 regard atoms as complex, and imagines them as being 

 composed of moving electrons. The electron is im- 

 material — it is the unit-charge of electricity. It is 

 said to possess mass, but mass is now understood to 

 mean inertia. So long as the electron is moving, it sets 

 up a field of energy round it, and this field — the electro- 

 magnetic one — extends in all directions. Periodic dis- 

 turbances in it constitute radiation, and this radiation 

 travels with the velocity of light. It is because of the 

 existence of this field that we are obliged to postulate 

 the existence of an ether of space. Unfamiliar to us 

 until the discovery of Hertzian waves and " wireless " 

 telegraphy, this electro-magnetic radiation in space is 

 now accessible to our direct intuitions. We can initiate 

 it by setting electrons in motion, that is, by expending 

 energy (producing the sparking in the transmitters of 

 the wireless telegraphy apparatus) ; and we can stop 

 it, if it is in existence, by absorbing the energy (in the 



