LIGHT, COLOR, AND SOUND 217 



stops and the other begins; they seem to merge; a flame 

 seems to be both light and heat. All these, and many 

 similar phenomena, indicate the close relationship of the 

 two forms of energy. 



What Light Is. Now we come to the question, what 

 is light ? Heat we found to be molecular motion, the heat 

 that we feel being the effect that this motion produces 

 on us. But did it occur to you to ask, when studying heat, 

 how, if it be molecular motion, does it get to us from the 

 sun through the tremendous spaces which are practically 

 empty, practically without molecules? Now you are in 

 a position to understand this. 



Heat, we can still say, is molecular movement wherever 

 there are molecules to move, and, since our personal experi- 

 ences are confined to spaces which are well filled with mole- 

 cules, this, for practical purposes, does pretty well as a 

 definition of heat. But we must also allow that heat 

 passes through space which is empty of molecules. This 

 we explain on the theory that heat is vibration, and that 

 its passage through empty space is as vibrations of the 

 ether. What ether is no one knows, but it is the physicists' 

 term for whatever it is that occupies empty (molecularly 

 empty) space. When these vibrations of the ether en- 

 counter the fringes of the atmosphere, they cause vibra- 

 tion of the atmospheric molecules. So we may properly 

 say that, from here on, heat is molecular motion. These 

 vibrations or motions which constitute heat are wave-like, 

 and these waves, by means of fine instruments, are actually 

 measurable as to their length and as to the speed with 

 which they travel. 



Have you ever tossed a stone into a pool of still water, 



