September 20, 1901.] 



SCIENCE. 



443 



wbicli has failed if it leaves anything un- 

 explained, which must be carried on indef- 

 initely on exactly the same principles, 

 whether the ultimate results are, or are 

 not, repugnant to common sense. 



Physical theories begin at the surface 

 with phenomena which directly affect our 

 senses. When they are used in the attempt 

 to penetrate deeper into the secrets of 

 nature it is more than probable that they 

 will meet with insuperable barriers, but 

 this fact does not demonstrate that the 

 fundamental assumptions are false, and the 

 question as to whether any particular ob- 

 stacle will be forever insuperable can rarely 

 be answered with certainty. 



Those who belittle the ideas which have 

 of late governed the advance of scientific 

 theory too often assume that there is no 

 alternative between the opposing assertions 

 that atoms and the ether are mere figments 

 of the scientific imagination, or that, on 

 the other hand, a mechanical theory of the 

 atoms and of the ether, which is now con- 

 fessedly imperfect, would, if it could be 

 perfected, give us a full and adequate rep- 

 resentation of the underlying realities. 



For my own part I believe that there is a 

 via media. 



A man peering into a darkened room, 

 and describing what he thinks he sees, may 

 be right as to the general outline of the ob- 

 jects he discerns, wrong as to their nature 

 and their precise forms. In his description 

 fact and fancy may be blended, and it may 

 be difficult to say where the one ends and 

 the other begins ; but even the fancies will 

 not be worthless if they are based on a 

 fragment of truth, which will prevent the 

 explorer from walking into a looking-glass 

 or stumbling over the furniture. He who 

 saw ' men as trees walking ' had at least a 

 perception of the fundamental fact that 

 something was in motion around him. 



And so, at the beginning of the twentieth 

 century, we are neither forced to abandon 



the claim to have penetrated below the 

 surface of nature, nor have we, with all 

 our searching, torn the veil of mystery from 

 the world around us. 



The range of our speculations is limited 

 both in space and time : in space, for we 

 have no right to claim, as is sometimes 

 done, a knowledge of the ' infinite universe '; 

 in time, for the cumulative effects of actions 

 which might pass undetected in the short 

 span of years of which we have knowledge, 

 may, if continued long enough, modify our 

 most profound generalizations. If some 

 such theory as the vortex-atom theory were 

 true, the faintest trace of viscosity in the 

 primordial medium would ultimately de- 

 stroy matter of every kind. It is thus a 

 duty to state what we believe we know in 

 the most cautious terms, but it is equally a 

 duty not to yield to mere vague doubts as 

 to whether we can know anything. 



If no other conception of matter is pos- 

 sible than that it consists of distinct phys- 

 ical units — and no other conception has 

 been formulated which does not blur what 

 are otherwise clear and definite outlines — 

 if it is certain, as it is, that vibrations 

 travel through space which cannot be prop- 

 agated by matter, the two foundations of 

 physical theory are well and truly laid. It 

 may be granted that we have not yet framed 

 a consistent image either of the nature of 

 the atoms or of the ether in which they exist; 

 but I have tried to show that in spite of the 

 tentative nature of some of our theories, in 

 spite of many outstanding difficulties, the 

 atomic theory unifies so many facts, sim- 

 plifies so much that is complicated, that we 

 have a right to insist — at all events till an 

 equally intelligible rival hypothesis is pro- 

 duced — that the main structure of our 

 theory is true ; that atoms are not merely 

 helps to puzzled mathematicians, but phys- 

 ical realities. 



Arthur W. Rtjcker. 



University of Loxdox. 



