166 SATURDAY LECTURES. 



is incalculable, and insinuates itself into the infinitesmally 

 minute interstices of the hardest known substances. It ex- 

 ists alike in a vacuum and in the substance of the diamond. 

 It is of such extreme tenuitj^ that a mass of it as large as 

 the earth weighs only a few grains. It is the least material 

 of all substances, and yet it possesses a tension or elasticity 

 far in excess of any matter of which we have any knowledge. 



This is the medium through which that form of force 

 we call light is supposed to act. It has, as you see, no ex- 

 istence whatever as a fact capable of demonstration by any 

 of the ordinary methods of proof It exists alone in that 

 highest faculty of the mind which, by its creative power, 

 sets man above the beasts of the field — the imagination. 



But our task does not end here. We have the medium, 

 but we have not the manner in which this is acted on to 

 produce the effect we know as light. Again the imagina- 

 tion must be brought into action. The mind which first 

 conceived of light as being a force, the result of wave mo- 

 tion, gave us the key which has unlocked some of the deep- 

 est mysteries of Nature, and made modern physical science 

 and all that belongs to it, and has resulted from it possible; 

 for th^ foundation of the physical science of the present 

 day is the fact that all energ}^ is but a mode of mo- 

 tion. I think it can be broadly stated that all forms of 

 energy, whether in the organic or inorganic world, are but 

 modes of motion. The ancient philosopher, when asked to 

 define Life, moved his arms, drew the air into his lungs, 

 and expelled it, and said, " This is life," meaning that all 

 we knew of life were its phenomena. But he explained bet- 

 ter than he was aware of. He expressed thus the idea 

 which underlies the modern concepts of Nature and her 

 laws, that life is motion. Without energy, as expressed by 

 some form of movement, there can be no life; and life is 

 but one form of energy, one particular kind of force. All 

 other forces in Nature are likewise but representations of 

 some kind of motion — they are not Things. The whole of the 

 teaching of modern science tends to the demonstration of 

 the fact that .there are but two Things in nature — matter and 

 its motion. 



