Septembee 21, 1900.] 



SCIENCE. 



423 



mainly concerned witli 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 uni- 

 form constitution of the intervening me- 

 dium in which they have their seat. The 

 other province is that in which the distri- 

 bution of the material molecules comes into 

 account. Setting aside the ordinary dy- 

 namics of matter in bulk, which is founded 

 on the uniformity of the properties of the 

 bodies concerned and their experimental 

 determination, we must assign to this re- 

 gion all phenomena which are concerned 

 with the uncoordinated motions of the 

 molecules, including the range of thermal 

 and in part of radiant actions ; the only 

 possible basis for detailed theory is the sta- 

 tistical dynamics of the distribution of the 

 molecules. The far more deep-seated and 

 mysterious processes which are involved in 

 changes in the constitution of the individ- 

 ual molecules themselves are mainly out- 

 side the province of physics, which is com- 

 petent 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 deter- 

 mine 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 do- 

 main of very precise knowledge relating, in 

 some diagrammatic manner, to the topog- 

 raphy 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 il- 

 lustration both of the rationality of natural 

 processes and of the analytical powers of 

 the human mind. In a word, the compli- 

 cation of the material world is referable to 



the vast range 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 in- 

 teractions at a distance between material 

 bodies to the part played by the sether is a 

 step towards the elimination of extraneous 

 and random hypotheses about laws of at- 

 traction between atoms. It also places 

 that medium on a different basis from mat- 

 ter, in that its mode of activity is simple 

 and regular, whereas intimate material in- 

 teractions must be of illimitable complexity. 

 This gives strong ground for the view that 

 we should not be tempted towards explain- 

 ing 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 dynam- 

 ical correlation, just as geometry explores 

 or correlates, without explaining, the de. 

 scriptive 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 

 mechanical effect is transmitted in nature. 

 This doctrine implies an expectation that 

 we may ultimately discover something anal- 

 ogous 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 effect 

 by material frame work. 



At a time when the only definitely ascer- 

 tained function of the sether was the un- 

 dulatory propagation of radiant energy 

 across space, Lord Kelvin pointed out that, 

 by reason of the very great velocity of prop- 



