Septebiber 20, 1901.] 



SCIENCE. 



431 



Nor need the analysis stop at the first 

 step. Water vapor and carbonic 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 analysis 

 stop if we are to avoid crossing the bound- 

 ary between fact and fiction ? Is there any 

 fundamental difference between resolving 

 air into a mixture of gases and resolving 

 an elementary gas into a mixture of atoms 

 and ether ? 



There afe those who cry halt at the 

 point at which we divide a gas into mole- 

 cules, and their first objection seems to be 

 that molecules and atoms cannot be directly 

 perceived, cannot be 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 con- 

 tinuous masses separated hy circular rifts. 

 This is the phenomenon which is observed 

 through a telescope. By no known means 

 can we ever approach or handle the rings ; 

 yet everybody who understands the evi- 

 dence now believes that they are not what 

 they appear to be, but consist of minute 

 moonlets, closely packed indeed, but sepa- 

 rate the one from the other. 



In the first place Maxwell proved mathe- 

 matically 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 would move slowest, 

 while a satellite moves faster the nearer it 

 is to a planet. Now spectroscopic observa- 

 tion, based on the beautiful method of Sir 

 W. Huggins, shows not only that the in- 

 ner portions of the ring move the more 



rapidly, but that the actual velocities of 

 the outer and inner edges are in close ac- 

 cord with the theoretical velocities of sat- 

 ellites at like distances from the planet. 



This and a hundred similar cases prove 

 that it is possible to obtain convincing evi- 

 dence 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 con- 

 tinuous 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 consti- 

 tutes 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 cogni- 

 zance, 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. 



This point is so important that it requires 

 full discussion, but in dealing with it, it is 

 necessary to distinguish carefully between 

 the validity of the arguments which support 

 the earlier and more fundamental proposi- 

 tions of the theory ; and the evidence 

 brought forward to justify mere speculative 

 applications of its doctrines which might be 

 abandoned without discarding the theory 

 itself. The proof of the theory must be 

 carried out step by step. 



The first step is concerned wholly with 

 some of the most general properties of mat- 

 ter, and consists in the proof that those 

 properties are either absolutely unintel- 

 ligible, or that, in the case of matter of all 

 kinds, we are subject to an illusion similar 

 to that, the results of which we admit in the 



