made. But we can see more than this. So far we say that light consists of waves, merely in the sense that it consists of some phenomenon or other which is periodic in time and in place; but we know that a ray of light is capable of doing work. Radiant heat, for example, striking on a body, will warm it and enable it to do work by expansion; therefore this periodic phenomenon which takes place in the ray of light is something or other which possesses mechanical energy, which is capable of doing work. We may make it, if you like, a mere matter of definition, and say, "Any change which possesses energy is a motion of matter;" and this is perhaps the most intelligible definition of matter that we can frame. In that sense, and in that sense only, it is a matter of demonstration, and not a matter of guess, that light consists of the periodic motion of matter which is between the luminous object and our eyes. But that something is not matter in the ordinary sense of the term, it is not made up of such molecules as gases and liquids and solids are made up of. This last statement, again, is no guess, but a proved fact.
There are people who ask, "Why is it necessary to suppose a luminiferous ether to be any thing else except molecules of matter in space, in order to carry light about?" The answer is a very simple one. In order that separate molecules may carry about a disturbance, it is necessary that they should travel at least as fast as the disturbance travels. Now we know, by means that I shall afterward come to, that the molecules of gas travel at a very ordinary rate, about twenty times as fast as a good train. But, on the contrary, we know by the most certain of all evidence, by five or six different means, that the velocity of light is 200,000 miles a second. By that very simple means we are able to tell that it is quite impossible for light to be carried by the molecules of ordinary matter, and that it wants something else that lies between those molecules to carry the light. Now, remembering the evidence which we have for the existence of this ether, let us consider another piece of evidence, let us now consider what evidence we have that the molecules of a gas are separate from one another and have something between them. We find out by experiment, again, that the different colors of light depend upon the various rapidity of these waves, depend upon the size and upon the length of the waves that travel through the ether, and that when we send light through glass or any transparent medium except a vacuum, the waves of different lengths travel with different velocities. That is the case with the sea; we find that long waves travel faster than small ones. In much the same way, when light comes out of a vacuum and impinges upon any transparent medium, say upon glass, we find that the rate of transmission of all the light is diminished, that it goes slower when it gets inside of a material body; and that this change is greater in the case of large waves than of small ones. The small waves correspond to blue light and the large waves correspond
