TRANSACTIONS OF SECTION A. 617 



of sethereal transmission or in the more fortuitous domain of the statistics of 

 interacting molecules, are those around which attention is still mainly concentrated ; 

 but as the result of the progress in each, they are now tending towards consolida- 

 tion into one subject. I propose — leaving further review of the scientific aspect of 

 the recent enormous development of the applications of physical science for hands 

 more competent to deal with the practical side of that subject — to ofier some re- 

 marks 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 pronounced 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 de- 

 veloped theories which analysed all physical actions into direct attractions between 

 the separate elements of the bodies concerned, in favour 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 centuries, 

 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 relations ; 

 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 dynamical science. In England, 

 whence the innovation 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 physical action. 

 Although the pupil's imagination 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 framework into which it was built. The 

 more advanced reformers have now thrown overboard the apparatus of poten- 

 tial functions which Maxwell found necessary for the dynamical consolidation or 

 his theory, retaining only the final result as a verified descriptive 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 tenta- 

 tive method of describing the course of the transmission 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 indispensable 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 abstract physics is in fact 

 roughly divisible into two regions. In one of them we are mainly concerned with 

 interactions between one portion of matter and another portion occupying a dif- 

 ferent position in space ; such interactions have very uniform and comparatively 

 simple relations ; and the reason is traceable to the simple and uniform constitution 

 of the intervening medium in which they have their seat. The other province is 

 that in which the distribution of the material molecules comes into account. Set- 

 ting aside the ordinary dynamics of matter in bulk, which is founded on the 

 uniformity of the properties of the bodies concerned and their experimental deter- 

 mination, we must assign to this region all phenomena which are concerned with 

 the uncoordinated iiotions of the molecules, including the range of thermal and in 

 part of radiant actions ; the only possible basis for detailed theory is the statistical 

 dynamics of the distribution of the molecules. The far more deep-seated and 



