474 



NA TURE 



[September 12, 1901 



which none but experts are acquainted. It is forced upon us 

 at the very threshold of our study of Nature. Either the 

 properties of matter in bulk cannot be referred to any simpler 

 structure, or that simpler structure must have properties different 

 from those of matter in bulk as we directly knew it — properties 

 which can only be inferred from the results which they produce. 



No a p}-ioi-i argument against the possibility of our discover- 

 ing the existence of quasi-material substances, which are never- 

 theless different from matter, can prove the negative proposition 

 that such substances cannot exist. It is not a self-evident 

 truth that no substance other than ordinary matter can have an 

 existence as real as that of matter itself. It is not axiomatic 

 that matter cannot be composed of parts whose properties are 

 different from those of the whole. To assert that even if such 

 substances and such parts exist no evidence however cogent 

 could convince us of their existence is to beg the whole question 

 at issue ; to decide the cause before it has been heard. 



We must therefore adhere to the standpoint adopted by most 

 scientific men, viz., that the question of the existence of ultra- 

 physical entities, such as atoms and the ether, is to be settled by 

 the evidence, and must not be ruled out as inadmissible on a 

 priori grounds. 



On the other hand, it is impossible to deny that, if the mere 

 entry on the search for the concealed causes of physical pheno- 

 mena is not a trespass on ground we have no right to explore, 

 it is at all events the beginning of a dangerous journey. 



The wraiths of phlogiston, caloric, luminiferous corpuscles, 

 and a crowd of other phantoms haunt the investigator, and as 

 the grim host vanishes into nothingness he cannot but wonder 

 if his own conceptions of atoms and of the ether 



"shall dissolve, 



And, like this insubstantial p.lgeant faded, 



Leave not a wrack behind," 



But though science, like Bunyan's hero, has sometimes had 

 to pass through the "Valley of Humiliation," the spectres 

 which meet it there are not really dangerous if they are boldly 

 faced. The facts that mistakes have been made, that theories 

 have been propounded, and for a time accepted, which later 

 investigations have disproved, do not necessarily discredit the 

 method adopted. In scientific theories, as in the world around 

 us. there is a survival of the fittest, and Dr. James Ward's un- 

 sympathetic account of the blunders of those vvhose work, after 

 all, has shed glory on the nineteenth century, might mutatis 

 Diutandis stand for a description of the history of the advance of 

 civilisation. " The story of the progress so far," he tells us, " is 

 briefly this : Divergence between theory and fact one part of the 

 way, the wreckage of abandoned fictions for the rest, and an un- 

 attainable goal of phenomenal nihilism and ultra-physical 

 mechanism beyond" (" Naturalism and Agnosticism," vol. i. 



P- 153)- 



" The path of progress," says Prof. Karl Pearson, " is strewn 

 with the wreck of nations. Traces are es'erywhere to be seen 

 of the hecatombs of inferior races, and of victims who found not 

 the narrow way to the greater perfection. Yet these dead 

 peoples are, in very truth, the stepping-stones on which man- 

 kind has arisen to the higher intellectual and deeper emotional 

 life of to-day " ( " National Life from the Standpoint of Science," 

 p. 62). 



It is only necessary to add that the progress of society is 

 directed towards an unattainable goal of universal contentment, 

 to make the parallel complete. 



And so, in the one case as in the other, we may leave " the 

 dead to bury their dead." The question before us is not whether 

 we too may not be trusting to false ideas, erroneous experi- 

 ments, evanescent theories. No doubt we are ; but, without 

 iiiaking an insolent claim to be better than our fathers, we may 

 fairly contend that, amid much that is uncertain and temporary, 

 some of the fundamental conceptions, some of the root-ideas of 

 science, are so grounded on reason and fact that we cannot but 

 regard them as an aspect of the very truth. 



Enough has, perhaps, now been said on this point for my 

 immediate purpose. The argument as to the constitution of 

 matter could be developed further in the manner I have hitherto 

 adopted, viz. by series of propositions, the proof of each of which 

 is based upon a few crucial phenomena. In particular, if matter 

 is divided into moving granules or particles, the phenomenon of 

 cohesion proves that there must be mutual actions between them 

 analogous to those which take place between large masses of 

 matter, and which we ascribe to force, thereby indicating the 

 regular, unvarying operation of active machinery which we have 



NO. 1663, VOL. 64] 



not yet the means of adequately understanding. For the 

 moment, I do not wish to extend the line of reasoning that has 

 been followed. My main object is to show that the notion of 

 the existence of ultra-physical entities and the leading outlines 

 of the atomic theory are forced upon us at the beginning of our 

 study of Nature, not only by a /rw;7 considerations, but in the 

 attempt to comprehend the results of even the simplest observa- 

 tion. These outlines cannot be effaced by the difficulties which 

 undoubtedly arise in filling up the picture. The cogency of the 

 proof that matter is coarse-grained is in no way affected by the 

 fact that we may have grave doubts as to the nature of the 

 granules. Nay, it is of the first importance to recognise that, 

 though the fundamental assumptions of the atomic theory 

 receive overwhelming support from a number of more detailed 

 arguments, they are themselves almost of the nature of axioms, 

 in that the simplest phenomena are unintelligible if they are 

 abandoned. 



The Range of the Atomic Theory. 



It would be most unfair, however, to the atomic theory to 

 represent it as depending on one line of reasoning only, or to 

 treat its evidence as bounded by the very general propositions I 

 have discussed. 



It is true that as the range of the theory is extended the funda- 

 mental conception that matter is granular must be expanded and 

 filled in by supplementary hypotheses as to the constitution of 

 the granules. It may also be admitted that no complete or 

 wholly satisfactory description of that constitution can as yet be 

 given ; that perfection has not yet been attained here or in any 

 other branch of science ; but the number of facts which can be 

 accounted for by the theory is very large compared with the 

 number of additional hypotheses which are introduced ; and the 

 cumulative weight of the additional evidence obtained by the 

 study of details is such as to add greatly to the strength of the 

 conviction that, in its leading outlines, the theory is true. 



It was originally suggested by the facts of chemistry, and 

 though, as we have seen, a school of chemists now thrusts it into 

 the background, it is none the less true, in the words of Dr. 

 Thorpe, that "every great advance in chemical knowledge 

 during the Last ninety years finds its interpretation in [Dalton's] 

 theory" (" Essays on Historical Chemistry," 1894, p. 368). 



The principal mechanical and thermal properties of gases have 

 been explained, and in large part discovered, by the aid of the 

 atomic theory ; and, though there are outstanding difficulties, 

 they are, for the most part, related to the nature of the atoms 

 and molecules, and do not affect the question as to whether 

 they exist. 



The fact that different kinds of light all travel at the same 

 speed in interplanetary space, while they move at different 

 rates in matter, is explained if matter is coarse-grained. But 

 to attempt to sum up all this evidence would be to recite a text- 

 book on physics. It must suffice to say that it is enormous in 

 extent and varied in character, and that the atomic theory im- 

 parts a unity to all the physical sciences which has been attained 

 in no other way. 



I must, however, give a couple of instances of the wonderful 

 success which has been achieved in the explanation of physical 

 phenomena by the theory we are considering, and I select them 

 because they are in harmony with the line of argument I have 

 been pursuing. 



When a piece of iron is magnetised, its behaviour is different 

 according as the magnetic force applied to it is weak, moderate, 

 or strong. When a certain limit is passed, the iron behaves as 

 a non-magnetic substance to all further additions of magnetic 

 force. With strong forces it does and with very weak forces it 

 does not remain magnetised when the force ceases to act. Prof. 

 Ewing has imitated all the minute details of these complicated 

 properties by an arrangement of small isolated compass needles 

 to represent the molecules. It may fairly be said that as far as 

 this particular set of phenomena is concerned a most instructive 

 working model based on the molecular theory has not only been 

 imagined but constructed. 



The next illustration is no less striking. We may liken a 

 crowd of molecules to a fog ; but while the fog is admitted by 

 everybody to be made up of separate globules of water, the 

 critics of scientific method are sometimes apt to regard the 

 molecules as mere fictions of the imagination. If, however, we 

 could throw the molecules of a highly rarefied gas into such a 

 state that vapour condensed on them, so that each became the 

 centre of a water-drop, till the host of invisible molecules was, 

 as it were, magnified by accretion into a visible mist, surely no 



