10 REPORT— 1901. 



, Nor need the analysis stop at tlio first step. Water vapour and car- 

 bonic acid, themselves constituents of the atmosphere, are in turn resolved 

 into their elements hydrogen, oxygen, and carbon, which, without a 

 formal discussion of the criteria of reality, we may safely say are as real 

 as air itself. 



Now, at what point must this analy.sis stop if we are to avoid crossing 

 the boundary between fact and fiction ? Is there any fundamental differ- 

 ence between resohing air into a mixture of gases and resolving an 

 elementary gas into a mixture of atoms and ether 1 



There are those who cry halt ! at the point at which wo divide a gas 

 into molecules, and their first objection seems to be that molecules and 

 atoms cannot be directly perceived, cannot l^e seen or handled, and are 

 mere conceptions, which have their uses, but cannot be regarded as 

 realities. 



It is easiest to reply to this objection by an illustration. 

 The rings of Saturn appear to be continuous masses separated by 

 circular rifts. This is the phenomenon which is observed through a tele- 

 scope. By no known means can we ever approach or handle the rings ; 

 yet everybody who understands the evidence now believes that they are 

 not what they appear to be, but consist of minute moonlets, closely packed 

 indeed, but separate the one from the other. 



In the first plaoe Maxwell proved mathematically that if a Saturnian 

 ring were a continuous solid or fluid mass it would be unstable and would 

 necessarily break into fragments. In the next place, if it were possible for 

 the ring to revolve like a solid body, the inmost parts woidd move slowest, 

 while a satellite moves faster the nearer it is to a planet. Now spectro- 

 scopic observation, based on the beautiful method of Sir W. Huggins, 

 shows not only that the inner portions of the ring move the more 

 rapidly, but that the actual velocities of the outer and inner edges are 

 in close accord with the theoi'etical velocities of satellites at like distances 

 from the planet. 



This and a hundred similar cases prove that it is possible to obtain 

 convincing evidence of the constitution of bodies between whose separate 

 parts we cannot directly distinguish, and I take it that a physicist who 

 believes in the reality of atoms thinks that he has as good reason for 

 dividing an apparently continuous gas into molecules as he has for dividing 

 the apparently continuous Saturnian rings into satellites. If he is wrong 

 it is not the fact that molecules and satellites alike cannot be handled 

 and cannot be seen as individuals, that constitutes the difference between 

 the two cases. 



It may, however, be urged that atoms and the ether are alleged to have 

 properties different from those of matter in bulk, of which alone our senses 

 take direct cognisance, and that therefore it is impossible to prove their 

 existence by evidence of the same cogency as that which may prove the 

 existence of a newly discovered variety of matter or of a portion of matter 

 too small or too distant to be seen. 



