618 REPORT— 1900. 



mysterious processes which are involved in changes in the constitution of the indi- 

 vidual molecules themselves are mainly outside the province of pliysics, whiL-h 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 diagrammatic 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 wonderlul existing illustration both of the rationality of natural 

 processes and of the aualytical powers of the human mind. In a word, the com- 

 plication of the material world is referable to the vast r.mge of structure and of 

 states of aggregation in the material atoms ; while the possibility 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 distance between 

 material bodies to the part played by the agther 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 explaining the simple grcup of relations which have been found 

 to define the activity of the rether, 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 corre- 

 lates, without explaining, 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 which he transmits them 

 from one place to another, to be the archetype of the processes by which all mecha- 

 nical eftect is transmitted in nature. This doctrine implies an expectation that we 

 may ultimatelj' discover something analogous to structure in the celestial spaces, 

 by means of which the transmission of physical effect will be brought into line 

 with the transmission of mechanical eftect by material iramework. 



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 comparison with the 

 amount of energy that is transmitted in a second of time : this easily led him to the 

 very striking conclusion that, on the hypothesis that the aether is like material 

 elastic media, it is not necessary to assume its density to be more than 10"i^ 

 of that of water, or its optical rigidity to be more than ten 10'^ of that of steel 

 or glass. Thus far the aether would be merely an impalpable material atmosphere 

 for the transference ot energy by radiation, at extremely small densities but with very 

 great speed, while ordinary matter would be the seat of practically all this energy. 

 But this way of explaining the absence of sensible influence of the aether 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 



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

 illustrations of a solid a3ther interacting with material molecules and with itself by 

 attraction at a distance : unlike the generalised dynamical methods expounded in 

 the text, which can leave the intimate structure of the material molecule outside the 

 problem, a definite working constitution is there assigned to the molecular nucleus. 

 It is pointed out in a continuation that is to appear in the Phil. Mafj. for September, 

 that a density of sether of the order of only 10"^", which would not appreciably affect 

 the inertia of matter, would involve rigidity comparable with that of steel, and 

 thus permit transmission of magnetic forces by stress ; this solid rether is however, 

 as usual, taken to be freely permeable to the molecules of matter. 



