Page:Popular Science Monthly Volume 7.djvu/285

from that supposition what these consequences ought to be, and showing that these consequences are 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 distributed 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 different 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 displacement 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, the fact that light is a phenomenon, periodic 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