Januaby 18, 1918] 



SCIENCE 



55 



These make up a special order of phenom- 

 ena which can not be explained by the prin- 

 ciples of motion and equilibrium." His 

 attitude may fairly be taken as, in general, 

 characteristic of his time ; there were sharp 

 lines of demarkation between the different 

 departments of natural philosophy which 

 would doubtless cause feelings of surprise 

 and discomfort to a modern physicist if he 

 could suddenly find himself at a meeting of 

 the Roj'al Society or of the Paris Academy 

 in the year 1822. These barriers were to a 

 considerable extent broken down in the 

 forties by the discovery and development 

 of the principle of the conservation of 

 energy. It was not simply the quantitative 

 relation which excited the enthusiasm of 

 the men of that time, but the knowledge 

 that there was a "correlation of Physical 

 Forces"; their reception of the discovery 

 shows how much such a relation had been 

 wanted. 



The psychology of phj-sicists made it in- 

 evitable that energ3' should be regarded as 

 something more real than a mathematical 

 expression which remains constant during 

 various processes. It was given a quasi-sub- 

 stantial interpretation and localized in 

 space; and it was most natural that its 

 newly recognized forms should be identi- 

 fied as nearly as possible with the familiar 

 energy of ordinary mechanics. Thus we 

 had at once a mechanical theory of heat 

 which led to a great extension of molecular 

 hypotheses; and the desire to deduce the 

 empirical second law from dynamical prin- 

 ciples was the motive for the development 

 of statistical mechanics through the succes- 

 sive stages shown in the work of Maxwell, 

 Boltzmann and Gibbs. 



This tendency toward dynamical ex- 

 planations was strengthened by the prog- 

 ress of the wave-theory of light. After the 

 brilliant experiments and interpretations 



of Young and Fresnel it was impossible to 

 doubt the kineraatical similarity of light to 

 a transverse wave motion. This made it 

 necessary to postulate an ether and to give 

 it suitable properties; the theory of waves 

 in an ordinary material elastic solid waa 

 developed by Green, Cauehy, Thomson and 

 others and compared with the phenomena 

 of light. The lack of complete agreement 

 was a stimulus to the investigation of ocher 

 possible types of elastic substances con- 

 forming to the general laws of mechanics. 

 In the hands of MaeCullagh, Stokes, and 

 especially of Kelvin, these investigations 

 led to great advances in our knowledge of 

 the properties of continuous media, and 

 showed the dynamical possibility of the ex- 

 istence of media which were quite different 

 in their elastic properties from ordinary 

 matter. 



Another current of thought which in- 

 fluenced profoundly the complex develop- 

 ment of theoretical physics in the nine- 

 teenth century was the strong prejudice 

 of Faraday against action at a distance 

 and his instinctive preference for a mode 

 of representation which involved the trans- 

 fer of forces from point to point by the 

 interaction of contiguous parts of a con- 

 tinuous medium. The fertility and useful- 

 ness of this method in electromagnetism is 

 attested not only by Faraday's unparal- 

 leled success as a discoverer (for genius 

 choses the method best suited to itself), 

 but also by the fact that it has held the 

 field in elementary instruction as well as 

 in the most complicated applications of 

 electrical engineering. We all know how 

 Maxwell deliberately submitted himself to 

 the influence of this prejudice, and the 

 epoch-making result which followed from 

 its union with his mathematical skill. The 

 inclusion in a single theory of two great 

 bodies of phenomena, those of light and 

 those of electricity, was an achievement of 



