THE FIRST AND THE LAST CATASTROPHE. 271 



from that supposition what these consequences ought to be, and 

 showing that these consequences arc just the facts which we observe 

 instead of doing that, I say, we make certain experiments, we show 

 that certain facts are undoubtedly true, and from these facts we go 

 back by a direct chain of logical reasoning, which there is no way 

 of getting out of, to the statement that all matter is made up of 

 separate pieces or molecules, and that in matter of a given kind, in 

 oxygen, or in hydrogen, or in nitrogen, these molecules are of very 

 nearly the same weight, and have certain mechanical properties which 

 are common to all of them. In order to show you something of the 

 kind of evidence for that statement, I must mention another theory 

 which, as it seems to me, is in the same position, that is the doctrine 

 of the luminiferous ether, or that wonderful substance which is dis- 

 tributed all over space, and which carries light and radiant heat. By 

 means of certain experiments upon interference, we can show, not by 

 any hypothesis, not by any guess at all, but by a pure interpretation 

 of the experiment we can show that in every ray of light there is 

 some change or other, whatever it is, which is periodic in time and in 

 place. By saying it is periodic in time, I mean that at a given point 

 of the ray of light this change increases up to a certain point, then 

 decreases, then increases in the other direction, and then decreases 

 again. That is shown by experiments of interference ; it is not a 

 theory which will explain the facts, but it is a fact which is got out 

 of observation. By saying that this phenomenon is periodic in space, 

 I mean that, if at any given instant you could examine the ray of 

 light, you would find that some change or disturbance, whatever it 

 is, has taken place all along it in diflerent degrees. It vanishes at 

 certain points, and between these it increases gradually to a maximum 

 on one side and the other alternately. That is to say, in traveling 

 along a ray of light there is a certain change (which can be observed 

 by experiments, by operating upon a ray of light with other rays of 

 light), which goes through a periodic variation in amount. The 

 height of the sea, as you know if you travel along it, goes through 

 certain periodic changes ; it increases and decreases, and increases 

 and decreases again at definite intervals. And if you take the case 

 of waves traveling over the sea, and place yourself at a given point, 

 say you put a cork upon the surface, you will find that the cork will 

 rise up and down, that is to say, there will be a change or displace- 

 ment of the cork's position, which is periodic in time, which increases 

 and decreases, then increases in the opposite direction, and decreases 

 again. Now, this fact, which is established by experiment, and which 

 is not a guess at all, tlie fact that liffht is a phenomenon, pei'iodic in 

 time and space, is what we call the wave-theory of light. The word 

 theory here does not mean a guess ; it means an organized account 

 of the facts, such that from it you may deduce results, which may be 

 applicable to future experiments, the like of which have not yet been 



