TRANSACTIONS OF SECTION A. G19 



is to consider the .letber 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 sether 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 sethereal held 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 

 lai'ge compared with tho 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 largely con- 

 fined to average values. But the last half-century has witnessed the successful 

 application of a new instrument of research, which has removed in various direc- 

 tions the limitations that had previously been placed on the knowledge to which 

 it was possible for human eflbrt 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 individual molecule of matter, and provide a very definite and detailed, 

 though far from complete, analysis of the vibratory motions that are going on in it ; 

 these vibrations being in their normal state characteristic of its dynamical consti- 

 tution, and in their deviations from the normal giving indications of the velocity 

 of its movement and the physical state of its environment. Maxwell long ago laid 

 emphasis on the fact that a physical atomic theory is not competent even to con- 

 template the vast mass of potentialities and correlations of the past and the future, 

 that biological theory has to consider as latent in a single organic germ containing 

 at most only a few million molecules. On our present view we can accept his 

 position that the properties of such a body cannot he those of a ' purely material 

 system,' provided, however, we restrict this phrase to apply to physical properties 

 as here defined. But an exhaustive discovery of the intimate nature of the atom 

 is beyond the scope of physics ; questions as to whether it must not necessarily 

 involve in itself some image of the complexity of the organic structures of which 

 it can form a correlated part must remain a subject of speculation outside the 

 domain of that science. It might be held that this conception of discrete atoms 

 and continuous sether really stands, like those of space and time, in intimate 

 relation with our modes of mental apprehension, into which any consistent picture 

 of the external world must of necessity be fitted. In any case it would involve 

 abandonment of all the successful traditions of our subject if we ceased to hold 

 that our analysis can be formulatir-d in a consistent and complete manner, so far as 

 it goes, without being necessarily an exhaustive account of phenomena that are 

 beyond our range of experiment. Such phenomena may be more closely defined 

 as those connected with the processes of intimate combination of the molecules: 

 they include the activities of organic beings which all seem to depend on change 

 of molecular structure. 



If, then, we have so small a hold on the intimate nature of matter, it will appear 

 all the more striking that physicists have been able precisely to divine the mode of 

 operation of the intangible sether, and to some extent explore in it the fields of 

 physical influence of the molecules. On consideration we recognise that this 

 knowledge of fundamental physical interaction has been reached by a comparative 

 process. The mechanism of the propagation of light could never have been studied 



