6o CLASSICS OF MODERN SCIENCE 

 its original volume. This quality, considered along- with its penetra- 

 bility retained in spite of such condensation seems to show that it con- 

 sists of small particles that float about in rapid vibration in an ether 

 consisting of still more minute particles. Sound, then, is caused by 

 the struggle of these particles to escape when at any point in the 

 course of a wave they are more crowded together than at some other 

 point. 



Now the wonderful speed of light considered with its other quali- 

 ties, does not permit us to believe it to be transmitted in the same 

 manner. Therefore I shall try to explain the way in which I think it 

 must take place. I must first, however, describe that quality of hard 

 substances through which they transmit motion one to another If one 

 take a number of balls of the same size of any hard substance, and 

 place them touching one another in one line, he will find that on let- 

 ting a ball of the same size strike against one end of the line, the 

 motion is transmitted in an instant to the other end of the line. The 

 last ball is driven from the Hne while the others are apparently un- 

 disturbed, the ball that struck the line coming to a dead stop. This 

 is an illustration of a transmission of motion at great speed, varying 

 directly as the hardness of the bails. Yet it is certain that this trans- 

 mission is not instantaneous, but requires time. For if the move- 

 ment, or if you wish, the tendency to move, did not pass from one 

 ball to another in succession, they would all be set in motion at the 

 same instant and would all move forward at the same time. Now 

 this is so far from the case that only the last one leaves the row, and 

 it has the speed of the ball that first struck the line. 



There are other experiments, also demonstrating that all bodies, 

 even those thought hardest, such as steel, glass and agate, are really 

 elastic, and bend a little, no matter whether they are in rods, balls, 

 or bodies of any other shape, — that is, they give slightly at the point 

 where struck, and at once regain their former shape. Thus I have 

 discovered that in letting a glass or agate ball strike on a large, thick, 

 flat piece of the same substance the surface of which has been rough- 

 ened by the breath, the place where it strikes is shown by a circular 

 indentation that varies in size directly as the force of the blow. 

 This indicates that the materials give when struck and then fly back, — 

 an event that necessarily takes time. 



Now to apply such a motion to the explanation of light, there is 



