f8 FRAGMENTS OF SCIENCE 



in proportion to your power of exercising this faculty 

 aright, and of associating definite mental images with the 

 terms employed, will be the pleasure and the profit which 

 you will derive from this lecture. The outward facts of 

 nature are insufficient to satisfy the mind. We cannot be 

 content with knowing that the light and heat of the sun 

 illuminate and warm the world. We are led irresistibly to 

 inquire, "What is light, and what is heat?" and this 

 question leads us at once out of the region of sense into 

 that of imagination. 1 



Thus pondering, and questioning, and striving to sup- 

 plement that which is felt and seen, but which is incom- 

 plete, by something imfelt and unseen which is necessary 

 to its completeness, men of genius have in part discerned, 

 aot only the nature of light and heat, but also, through 

 them, the general relationship of natural phenomena. The 

 working power of Nature consists of actual or potential 

 motion, of which all its phenomena are but special forms. 

 This motion manifests itself in tangible and in intangible 

 matter, being incessantly transferred from one to the other, 

 and incessantly transformed by the change. It is as real 

 in the waves of the ether as in the waves of the sea; the 

 latter derived as they are from winds, which in their turn 

 are derived from the sun are, indeed, nothing more than 

 the heaped -up motion of the ether waves. It is the calo- 

 rific waves emitted by the sun which heat our air, produce 

 our winds, and hence agitate our ocean. And whether 

 they break in foam upon the shore, or rub silently against 

 the ocean's bed, or subside by the mutual friction of their 



1 This line of thought was pursued further five years subsequently. See 

 "Scientific Use of the Imagination" in VoL IL 



