September 6, 19CX)] 



NATURE 



451 



■per subjects by a method of direct description of the mere 

 irse of phenomena, apart from any attempt to establish causal 

 hitions ; the initiation of this method being traced, like that of 



I Newtonian dynamics itself, to this country. The question 

 - arisen as to how far the new methods of rethereal physics 



to be considered as an independent departure, how far they 

 iin the natural development of existing dynamical science. In 

 .gland, whence the innovation came, it is the more conservative 

 -ition that has all along been occupied. Maxwell was himself 

 ined in the school of physics established in this country by 

 r George Stokes and Lord Kelvin, in which the dominating 

 \ has been that of the strictly dynamical foundation of all 

 ysical action. Although the pupil's imagination bridged over 

 namical chasms, across which the master was not always able 

 follow, yet the most striking feature of Maxwell's scheme was 



II the dynamical framework into which it was built. The 

 ..lore advanced reformers have now thrown overboard the 

 apparatus of potential functions which Maxwell found necessary 

 for the dynamical consolidation of 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 phenome- 

 non ; 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 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 oether, 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 specula- 

 tion, 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 different 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. Setting aside the ordinary 

 dynamics of matter in bulk, which is founded on the uniformity 

 of the pioperties of the bodies concerned and their experimental 

 determination, we must assign to this region all phenomena 

 which are concerned with the unco-ordinated motions 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 mysterious processes which are in- 

 volved in changes in the constitution of the individual molecules 

 themselves are mainly outside the province of physics, which is 

 competent to reason only about permanent material systems ; 

 they must be left to the sciences of chemistry and physiology. 

 Yet the chemist proclaims that he can determine only the 

 results of his reactions and the physical conditions under which 

 they occur ; the character of the bonds which hold atoms in 

 their chemical combinations is at present unknown, although 

 a large domain of very precise knowledge relating, in some dia- 

 grammatic manner, to the topography of the more complex 

 molecules has been attained. The vast structure which chemical 

 science has in this way raised on the narrow foundation of the 

 atomic theory is perhaps the most wonderful existing illustration 

 both of the rationality of natural processes and of the analytical 

 powers of the human mind. In a word, the complication of 

 the material world is referable to the vast range of structure and 

 of states of aggregation in the material atoms ; while the possi- 

 bility of a science of physics is largely due to the simplicity of 

 constitution of the universal medium through which the individual 

 atoms interact on each other. 



The reference of the uniformity in the interactions at a dis- 

 tance between material bodies to the part played by the lether 

 is a step towards the elimination of extraneous and random 

 hypotheses about laws of attraction between atoms. It also 

 places that medium on a different basis from matter, in that its 

 mode of activity is simple and regular, whereas intimate material 

 interactions must be of illimitable complexity. This gives strong 

 ground for the view that we should not be tempted towards ex- 



XO. 1610, VOL. 62] 



plaining the simple group of relations which have been found to 

 define the activity of the sether, by treating them as mechanical 

 consequences of concealed structure in that medium ; we should 

 rather rest satisfied with having attained to their exact dynamical 

 correlation, just as geometry explores or correlates, without ex- 

 plaining, the descriptive and metric properties of space. On 

 the other hand, a view is upheld which considers the pressures 

 and thrusts of the engineer, and the strains and stresses in the 

 material structures by whicli he transmits them from one place 

 to another, to be the archetype of the processes by which all 

 mechanical effect is transmitted in nature. This doctrine im- 

 plies an expectation that we may ultimately discover something 

 analogous to structure in the celestial spaces, by means of which 

 the transmission of physical eflFect will be brought into line with 

 the transmission of mechanical effect by material framework. 



At a time when the only definitely ascertained function of the 

 aether was the undulatory propagation of radiant energy across 

 space, Lord Kelvin pointed out that, by reason of the very 

 great velocity of propagation, the density of the radiant energy 

 in the medium at any place must be extremely small in com- 

 parison with the amount of energy that is transmitted m a 

 second of time : this easily led him to the very striking con- 

 clusion that, on the hypothesis that the aether is like material 

 elastic media, it is not necessary to assume its density to be 

 more than lO"'* of that of water, or its optical rigidity to be 

 more than ten lo"*^ of that of steel or glass. Thus far the rether 

 would be merely an impalpable material atmosphere for the 

 transference of energy by radiation, at extremely small densities 

 but with very great speed, while ordinary matter would be the 

 seat of practically all this energy. Hut this way of explaining 

 the absence of sensible influence of the a-ther on the phenomena 

 of material dynamics lost much of its basis as soon as it was 

 recognised that the same medium must be the receptacle of 

 very high densities of energy in the electric fields around 

 currents and magnets.^ The other mode of explanation is to 

 consider the aiher to be of the very essence of all physical 

 actions, and to correlate the absence of obvious mechanical 

 evidence of its intervention with its regularity and universality. 



On this plan of making the eether the essential factor in the 

 transformation of energy as well as its transmission across space, 

 the material atom must be some kind of permanent nucleus that 

 retains around itself an aethereal field of physical influence, such 

 as, for example, a field of strain. We can recognise the atom 

 only through its interactions with other atoms that are so far 

 away from it as to be practically independent systems ; thus our 

 direct knowledge of the atom will be confined to this field of 

 force which belongs to it. Just as the exploration of the distant 

 field of magnetic influence of a steel magnet, itself concealed 

 from view, cannot tell us anything about the magnet except the 

 amount and direction of its moment, so a practically complete 

 knowledge of the field of physical influence of an atom might be 

 expressible in terms of the numerical values of a limited number 

 of physical moments associated with it, without any revelation 

 as to its essential structure or constitution being involved. This 

 will at any rate be the case for ultimate atoms if, as is most 

 likely, the distances at which they are kept apart are large com- 

 pared with the diameters of the atomic nuclei ; it in fact forms 

 our only chance for penetrating to definite dynamical views of 

 molecular structure. So long as we cannot isolate a single 

 molecule, but must deal observationally with an innumerable 

 distribution of them, even this kind of knowledge will be largel 

 confined to average values. But the last half-century has wit- 

 nessed the successful application of a new instrument of re- 

 search, which has removed in various directions the limitations 

 that had previously been placed on the knowledge to which it 

 was possible for human effort to look forward. The spectroscope 

 has created a new astronomy by revealing the constitutions and 

 the unseen internal motions of the stars. Its power lies in the 

 fact that it does take hold of the internal relations of the indi- 

 vidual molecule of matter, and provide a very definite and 



1 We can here only allude to Lord Kelvin's recent most interesting 

 mechanical illustrations of a solid aether interacting with material molecules 

 and with itself by attraction at a distance : unlike the generalised dyna- 

 mical methods expounded in the text, which can leave the intimate 

 structure of the material molecule outside the problem, a definite working 

 constiiution is there assigned to the molecular nucleus. It is pointed out 

 in a continuation that is to appear in the Phil. Ma^. for September, that 

 a density of aether of the order of only lo ", which would not appreciably 

 affect the inertia of matter, would involve rigidity comparable with that of 

 stee', and thus permit transmission of magnetic forces by stress ; this solid 

 a;ther is, however, as usual, taken to be freely permeable to the molecules 

 of matter. 



