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SCIENCE. 



[N. S. Vol XII. No. 299. 



had come clearly into view a quarter of 

 a century ago, under Maxwell's lead, 

 whether in the exact dynamical relations 

 of eethereal transmission or in the more for- 

 tuitous domain of the statistics of interact- 

 ing molecules, are those around which at- 

 tention is still mainly concentrated ; but as 

 the result of the progress in each, they are 

 now tending towards consolidation into one 

 subject. I propose — leaving further review 

 of the scientific aspect of the recent enor- 

 mous development of the applications of 

 physical science for hands more competent . 

 to deal with the practical side of that sub- 

 ject — to ofifer some remarks on the scope 

 and validity of this molecular order of ideas, 

 to which the trend of physical explanation 

 and development is now setting in so pro- 

 nounced a manner. 



If it is necessary to offer an apology for 

 detaining the attention of the Section on so 

 abstract a topic, I can plead its intrinsic 

 philosophical importance. The hesitation 

 so long felt on the Continent in regard to 

 discarding the highly developed theories 

 which analyzed all physical actions into di- 

 rect attractions between the separate ele- 

 ments of the bodies concerned, in favor of a 

 new method in which our ideas are carried 

 into regions deeper than the phenomena, 

 has now given place to eager discussion of 

 the potentialities of the new standpoint. 

 There has even appeared a disposition to 

 consider that the Newtonian dynamical 

 principles, which have formed the basis of 

 physical explanation for nearly two cen- 

 turies, must be replaced in these deeper 

 subjects by a method of direct description 

 of the mere course of phenomena, apart 

 from any attempt to establish causal rela- 

 tions; the initiation of this method being 

 traced, like that of the Newtonian dynamics 

 itself, to this country. The question has 

 arisen as to how far the new methods of 

 sethereal physics are to be considered as an 

 independent departure, how far they form 



the natural development of existing dynam- 

 ical science. In England, whence the inno- 

 vation came, it is the more conservative 

 position that has all along been occupied. 

 Maxwell was himself trained in the school of 

 physics established in this country by Sir 

 George Stokes and Lord Kelvin, in which 

 the dominating idea has been that of the 

 strictly dynamical foundation of all phys- 

 ical action. Although the pupil's imagina- 

 tion bridged over dynamical chasms, across 

 which the master was not always able to 

 follow, yet the most striking feature of Max- 

 well's scheme was still the dynamical frame- 

 work into which it was built. The more 

 advanced reformers have now thrown over- 

 board the apparatus of potential functions 

 which Maxwell found necessary for the dy- 

 namical consolidation of his theory, retain- 

 ing only the final result as a verified de- 

 scriptive basis for the phenomena. In this 

 way all difficulties relating to dynamical 

 development and indeed consistency are 

 avoided, but the question remains as to how 

 much is thereby lost. In practical electro- 

 magnetics the transmission of power is 

 now the most prominent phenomenon ; if 

 formal dynamics is put aside in the general 

 theory, its guidance must here be replaced 

 by some more empirical and tentative 

 method of describing the course of trans- 

 mission and transformation of mechanical 

 energy in the system. 



The direct recognition in some form, 

 either explicitly or tacitly, of the part 

 played by the aether has become indispen- 

 sable to the development and exposition of 

 general physics ever since the discoveries of 

 Hertz left no further room for doubt that this 

 physical scheme of Maxwell was not merely 

 a brilliant speculation, but constituted, in 

 spite of outstanding gaps and difficulties, a 

 real formulation of the underlying unity in 

 physical dynamics. The domain of ab- 

 stract physics is in fact roughly divisible 

 into two regions. In one of them we are 



