472 



NA TURE 



[September 12, 1901 



of the properties of inanimate Nature, but should have secured a 

 mathematical description of her operations. 



There is no doubt that this is the easiest path to follow. Criti- 

 cism is avoided if we admit from the first that we cannot go 

 below the surface ; cannot know anything about the constitution 

 of material bodies ; but must be content with formulating a 

 description of their behaviour by means of laws of Nature ex- 

 pressed by equations. 



But it this is to be the end of the study of Natuie, it is evident 

 that the construction of the model is not an essential part of the 

 process. The model is used merely as an aid to thinking ; and 

 if the relations of phenomena can be investigated without it, so 

 much the better. The highest form of theory — it maybe said — 

 the widest kind of generalisation, is that which has given up the 

 attempt to form clear mental pictures of the constitution of 

 matter, which expresses the facts and the laws by language and 

 symbols which lead to results that are true, whatever be our view 

 as to the real nature of the objects with which we deal. From 

 this point of view the atomic theory becomes not so much false 

 as unnecessary ; it may be regarded as an attempt to give an 

 unnatural precision to ideas which are and must be vague. 



Thus, when Rumford found that the mere friction of metals 

 produced heat in unlimited quantity, and argued that heat was 

 therefore a mode of motion, he formed a clear mental picture of 

 what he believed to be occurring. But his experiments may be 

 quoted as proving only that energy can be supplied to a body in 

 indefinite quantity, and when supplied by doing work against 

 friction it appears in the form of heat. 



By using this phraseology we exchange a vivid conception 

 of moving atoms for a colourless statement as to heat energy, 

 the real nature of which we do not attempt to define ; and 

 methods which thus evade the problem of the nature ol the 

 things which the symbols in our equations rei>resent have been 

 prosecuted with striking success, at all events within the 

 range of a limited class of phenomena. A great school of 

 chemists, building upon the thermodynamics of Willard Gibbs 

 and the intuition of Van t' Hoff, have shown with wonderful 

 skill that, if a sufficient number of the data of experiment are 

 assumed, it is possible, by the aid of thermodynamics, to trace 

 the form of the relations between many physical and chemical 

 phenomena without the help of the atomic theory. 



But this method deals only with matter as our coarse senses 

 know it ; it does not pretend to penetrate beneath the surface. 



It is therefore with the greatest respect for its authors, and 

 with a full recognition of the enormous power of the weapons 

 employed, that I venture to assert that the exposition of such a 

 system of tactics cannot be regarded as the last word of science 

 in the struggle for the truth. 



Whether we grapple with them, or whether we shirk them : 

 however much or however little we can accomplish without 

 answering them, the questions still force themselves upon us : 

 Is matter what it seems to be ? Is interplanetary space full or 

 empty .'' Can we argue back from the direct impressions of our 

 senses to things which we cannot directly perceive ; from the 

 phenomena displayed by matter to the constitution of matter 

 itself? 



It is these questions which we are discussing to-night, and 

 we may therefore, as far as the present address is concerned, put 

 aside, once for all, methods of scientific exposition in which an 

 attempt to forma mental picture of the constitution of matter is 

 practically abandoned, and devote ourselves to the inquiries 

 whether the effort to form such a picture is legitimate, and 

 whether we have any reason to believe that the sketch which 

 science has already drawn is to some extent a copy, and not a 

 mere diagram, of the truth. 



Successive Steps in the Analysis of Matter. 



In dealing, then, with the question of the constitution of 

 matter and the possibility of representing it accurately, we may 

 grant at once that the ultimate nature of things is, and must 

 remain, unknown ; but it does not follow that immediately 

 below the complexities of the superficial phenomena which 

 affect our senses there may not be a simpler machinery of the 

 existence of which we can obtain evidence, indirect indeed, but 

 conclusive. 



The fact that the apparent unity which we call the atmosphere 

 can be resolved into a number of different gases is admitted ; 

 though the ultimate nature of oxygen, nitrogen, aigon, carbonic 

 acid, and water vapour is as unintelligible as that of air as a 



NO. 1663, VOL. 64] 



whole, so that the analysis of air may be said to have substituted 

 many incomprehensibles for one. 



Nobody, however, looks at the question from this point of 

 view. It is recognised that an investigation into the proximate 

 constitution of things may be useful and successful, even if their 

 ultimate nature is beyond our ken. 



Nor need the analysis stop at the first step. Water vapour 

 and carbonic acid, themselves constituents of the atmosphere, 

 are in turn resolved into their elements, hydrogen, o.xygen, 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 boundary 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 are those who cry halt ! at the point at which we divide 

 a gas into molecules, 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 continuous masses separated 

 by 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 evidence 

 now believes that they are not what they appear to be, but con- 

 sist of minute moonlets, closely packed indeed, but separate the 

 one from the other. 



In the first place, 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 would move slowest, while a satellite 

 moves faster the nearer it is to a planet. Now, spectroscopic 

 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 theoretical 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 can- 

 not 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. 



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 propositions 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 matter, and consists in the proof that 

 those properties are either absolutely unintelligible, 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 case of 

 Saturn's rings, clouds, smoke, and a number of similar instances. 

 The believer in the atomic theory asserts that matter exists in a 

 particular state ; that it consists of parts which are separate and 

 distinct the one from the other, and as such are capable of 

 independent movements. 



Up to this point no question arises as to whether the separate 

 parts are, like grains of sand, mere fragments of matter ; or 

 whether, though they are the bricks of which matter is built, 

 they have, as individuals, properties different from those of 

 masses of matter large enough to be directly perceived. If they 



