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274 does not belong to it as a whole, but belongs to the separate parts of it, for this reason: that if you squeeze the gas you do not alter the time of vibration. Let us suppose that we have a great number of fiddles in a room which are all in contact, and have strings accurately tuned to vibrate to certain notes. If you sang one of those notes all the fiddles would answer; but if you compress them you clearly put them all out of tune. They are all in contact, and they will not answer to the tune with the same precision as before. But if you have a room which is full of fiddles placed at a certain distance from one another, then if you bring them within shorter distances of one another, so that they still don't touch, they will not be put out of tune, they will answer exactly to the same note as before. We see, therefore, that since compression of a gas within certain limits does not alter the rate of vibration which belongs to it, that rate of vibration cannot belong to the body of gas as a whole, but it must belong to the individual parts of it. Now by such reasoning as this it seems to me that the modern theory of the constitution of matter is put upon a basis which is absolutely independent of hypothesis. The theory is simply an organized statement of the facts, a statement, that is, which is rather different from the experiments, being made out from them in just such a way as to be most convenient for finding out from them what will be the results of other experiments. That is all we mean at present by scientific theory.

Upon this theory Prof. Clerk Maxwell founded a certain argument in his lecture before the British Association at Bradford. It is a consequence of the molecular theory, as I said before, that all the molecules of a certain given substance, say oxygen, are as near as possible alike in two respects—first in weight, and secondly in their times of vibration. Now Prof. Clerk Maxwell's argument was this: He first of all said that the theory required us to believe not that these molecules were as near as may be alike, but that they were exactly alike in these two respects—at least the argument appeared to me to require that. Then he said all the oxygen we know of, whatever processes it has gone through—whether it is got out of the atmosphere, or out of some oxide of iron or carbon, or whether it belongs to the sun, or the fixed stars, or the planets, or the nebulæ—all this oxygen is alike. And all these molecules of oxygen we find upon the earth must have existed unaltered, or unappreciably unaltered, during the whole of the time the earth has been evolved. Whatever vicissitudes they have gone through, how many times they have entered into combination with iron or silver and been melted down beneath the crust of the earth, or deoxidized and sent up again through the atmosphere, they have remained steadfast to their original form unaltered, the monuments of what they were when the world began. Now, Prof. Clerk Maxwell argues that things which are unalterable, and are exactly alike, cannot have been formed by any natural