274 ^^^^ POPULAR SCIENCE MONTHLY. 



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 pi*ecision 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 vibra- 

 tion 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 argu- 

 ment 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 cai'bon, or whether it be- 

 longs to the sun, or the fixed stars, or the planets, or the nebulse 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 en- 

 tered into combination with iron or silver and been melted down 

 beneath the crust of the earth, or deoxidized and sent iTp again 

 through the atmosphere, they have remained steadfast to their origi- 

 nal form unaltered, the monuments of what thpy were when the world 

 began. Now, Prof. Clerk Maxwell argues that things which are unal- 

 terable, and are exactly alike, cannot have been formed by any natural 



