January 12, 190G.] 



SCIENCE. 



51 



it had come to be regarded as the only pos- 

 sible means of describing and discussing the 

 actions of nature. The gradual abandon- 

 ment of this position and the change to the 

 modern view according to which all ac- 

 tions in nature are transmitted through a 

 continuous medium and require time for 

 their transmission was accomplished only 

 after a long struggle that occupied the 

 greater part of the nineteenth century. 



The more or less conscious part taken 

 in this struggle by technical mechanics, 

 which in the same period developed into a 

 science, has not always been insisted upon 

 sufficiently. Technical mechanics has 

 always been free of the idea of central 

 forces. To the engineer the idea of forces 

 acting at a distance is completely foreign, 

 in spite of the curious fact that, until not 

 so very long ago, the typical example of 

 such a force, gravitation, was almost the 

 only force with which he had to deal. The 

 development of thermodynamics, which has 

 given us the principle of the conservation 

 of energy in its broadest aspect, was closely 

 connected with the rise of technical me- 

 chanics, but proceeded rather independ- 

 ently of the development of the other 

 branches of mathematical ' physics. Its 

 fundamental principles are of a very gen- 

 eral and abstract nature, and even where 

 the molecular hypothesis is well worked 

 out, as in the kinetic theory of gases, the 

 idea of central forces is in no way essential. 



Hydrodynamics, elasticity, optics, elec- 

 tricity and magnetism, though originally 

 based on molecular hypotheses and the 

 idea of central forces, in the course of their 

 development found themselves more or less 

 independent of these notions. In all of 

 them the important common feature is the 

 propagation of actions through a medium 

 which can be regarded, at least in first ap- 

 proximation, as continuous. In hydro- 

 dynamics and in the theory of elasticity 

 this medium is that unknown something 



which we call matter; in optics, and later 

 in the theory of electricity and magnetism, 

 it was found necessary to postulate the ex- 

 istence of another medium, the ether. 



It is well known how the ideas of Fara- 

 day, of Maxwell, of Hertz, gradually 

 gained ascendency over the older views and 

 led to the abandonment of the idea of 

 central forces acting instantaneously at a 

 distance, in almost all branches of physics 

 except in the theory of gravitation. It is 

 also known that Maxwell, by a brilliant 

 analysis, succeeded in establishing the con- 

 nection between his electromagnetic theory 

 and the analytical mechanics of Lagrange. 

 Thus, at the end of the nineteenth century 

 we find a general attitude toward physical 

 phenomena essentially different from that 

 prevailing at the end of the eighteenth 

 century. 



With the rise of the electron theory in the 

 course of the last tAventy-five years a new 

 element has been introduced into this de- 

 velopment, an element which seems des- 

 tined to affect very radically not only our 

 interpretation of physical phenomena, but 

 also our general views about the principles 

 of theoretical mechanics. The idea of the 

 electron has grown out of the idea of ions 

 as used in electrolysis. Each molecule of 

 an electrolyte may break up into two ions, 

 i. e., two atoms, or groups of atoms, carry- 

 ing equal and opposite charges. The 

 current passing through the electrolyte 

 then consists in the actual transfer of these 

 ions to the cathode and anode to which 

 they give up their charges. In his Fara- 

 day lecture, delivered in 1881, which marks 

 an epoch in the ion theory, Helmholtz says : 

 "If we accept the hypothesis that the ele- 

 mentary substances are composed of atoms, 

 we can not avoid concluding that electricity 

 also, positive as well as negative, is divided 

 into definite elementary portions, which 

 behave like atoms of electricity." 



These 'atoms of electricity,' since en- 



