Page:Popular Science Monthly Volume 7.djvu/287

to red light. The waves of red light are not made to travel so slowly as the waves of blue light, but, as in the case of waves traveling over the sea, when light moves in the interior of a transparent body the large waves travel quickest. Well, then, by using such a body as will separate out the different colors—a prism—we are able to affirm what are the constituents of the light which strikes upon it. The light that comes from the sun is made up of waves of various lengths; but making it pass through a prism we can separate it out into a spectrum, and in that way we find a band of light instead of a spot coming from the sun, and to every band in the spectrum corresponds a wave of a certain definite length and definite time in vibration. Now we come to a very singular phenomenon. If you take a gas such as chlorine and interpose it in the wave of that light, you will find that certain particular rays of the spectrum are absorbed, while others are not. Now, how is it that certain particular rates of vibration can be absorbed by this chlorine gas while others are not? That happens in this way, that the chlorine gas consists of a great number of very small structures, each of which is capable of vibrating internally. Each of these structures is complicated, and is capable of a change of relative position among its parts of a vibratory character. We know that molecules are capable of such vibrations, such internal vibrations, for this reason, that if we heat any solid body sufficiently it will in time give out light; that is to say, the molecules are got into such a state of vibration that they start the ether vibrating, and they start the ether vibrating at the same rate at which they vibrate themselves. So that what we learn from the absorption of certain particular rays of light by chlorine gas, is that the molecules of that gas are structures which have certain natural rates of vibration, precisely those rates of vibration which belong to the molecules naturally. If you sing a certain note to a string of a piano, that string if in tune will vibrate. If, therefore, a screen of such strings were put across a room, and you sang a note on one side, a person on the other side would hear the note very weakly or not at all, because it would be absorbed by the strings; but if you sang another note, not one to which the strings naturally vibrated, then it would pass through, and would not be eaten up by setting the strings vibrating. Now this question arises. Let us put the molecules aside for a moment. Suppose we do not know of their existence, and say, "Is this rate of vibration, which naturally belongs to the gas, a thing which belongs to it as a whole, or does it belong to separate parts of it?" You might suppose that it belongs to the gas as a whole. A jar of water, if you shake it, has a perfectly definite time in which it oscillates, and that is very easily measured. That time of oscillation belongs to the jar of water as a whole. It depends upon the weight of the water, and the shape of the jar. But now, by a very certain method, we know that the time of vibration which corresponds to a certain definite gas