September 29, 1905.] 



SCIENCE. 



395 



which Maxwell was able to annex the phe- 

 nomena of light to electricity. The meth- 

 ods by which Maxwell arrived at his great 

 discoveries are not generally admitted as 

 logically binding. Most physicists prefer 

 to regard them as an invaluable possession 

 as yet unliquidated in logical coin; but of 

 the truth of his equations there is no doubt. 

 Maxwell's theory has been frequently ex- 

 pounded by other great thinkers, by Ray- 

 leigh (1881), by Poincare (1890), by 

 Boltzmann (1890), by Heaviside (1889), 

 by Hertz (1890), by Lorentz and others. 

 Hertz and Heaviside, in particular, have 

 condensed the equations into the symmet- 

 rical form now commonly used. Poynting 

 (1884) contributed his remarkable theorem 

 on the energy path. 



Prior to 1870 the famous law of Weber 

 (1846) had gained wide recognition, con- 

 taining as it did Coulomb's law. Ampere's 

 law, Laplace's law, Neumann's law of in- 

 duction, the conditions of electric oscilla- 

 tion and of electric convection. Every phe- 

 nomenon in electricity was deducible from 

 it compatibly with the doctrine of the con- 

 servation of energy. Clausius (1878), 

 moreover, by a logical effort of extraordi- 

 nary vigor established a similar law. More- 

 over, the early confirmation of Maxwell's 

 theory in terms of the dielectric constant 

 and refractive index of the medium was 

 complex and partial. Rowland's (1876, 

 1889) famous experiment of electric con- 

 vection, which has recently been repeat- 

 edly verified by Pender and Cremieu and 

 others, though deduced from Maxwell's 

 theory, is not incompatible with "Weber's 

 view. Again the ratio between the electro- 

 static and the electromagnetic system of 

 units, repeatedly determined from the early 

 measurement of Maxwell (1868) to the 

 recent elaborate determinations of Abra- 

 ham (1892) and Margaret Maltby (1897), 

 with an ever closer approach to the velocity 



of light, was at its inception one of the 

 great original feats of measurement of 

 Weber himself associated with Kohlrausch 

 (1856). The older theories, however, are 

 based on the so-called action at a distance 

 or on the instantaneous transmission of 

 electromagnetic force. Maxwell's equa- 

 tions, while equally universal with the pre- 

 ceding, predicate not merely a finite time 

 of transmission, but transmission at the 

 rate of the velocity of light. The triumph 

 of this prediction in the work of Hertz has 

 left no further room for reasonable dis- 

 crimination. 



As a consequence of the resulting enthu- 

 siasm, perhaps, there has been but little 

 reference in recent years to the great in- 

 vestigation of Helmholtz (1870, 1874), 

 which includes Maxwell's equations as a 

 special case ; nor to his later deduction 

 (1886, 1893) of Hertz's equations from 

 the principle of least action. Nevertheless, 

 Helmholtz 's electromagnetic potential is 

 deduced rigorously from fundamental prin- 

 ciples and contains, as Duhem (1901) 

 showed, the electromagnetic theory of light. 



Maxwell's own vortex theory of physical 

 lines of force (1861, 1862) probably sug- 

 gested his equations. In recent years, how- 

 ever, the efforts to deduce them directly 

 from apparently simpler properties of a 

 continuous medium, as for instance from 

 its ideal elastics, or again from a special- 

 ized ether, have not been infrequent. Kel- 

 vin (1890) with his quasi-rigid ether, 

 Boltzmann (1893), Sommerfeld (1892) 

 and others have worked efficiently in this 

 direction. On the other hand, J. J. Thom- 

 son (1891, et seq.), with remarkable intui- 

 tion, affirms the concrete physical existence 

 of Faraday tubes of force, and from this 

 hypothesis reaches many of his brilliant 

 predictions on the nature of matter. 



As a final commentary on all these divers 

 interpretations, the important dictum of 



