PROGRESS IN NINETEENTH CENTURY 63 



Laplace's law, Neumann's law of induction, the conditions of electric 

 oscillation and of electric convection. Every phenomenon in electric- 

 ity was deducible from it compatibly with the doctrine of the con- 

 servation of energy. Clausius (1878), moreover, by a logical effort of 

 extraordinary vigor, established a similar law. Moreover, the early 

 confirmation of Maxwell's theory in terms of the dielectric constant 

 and refractive index of the medium was complex and partial. Row- 

 land's (1876, 1889) famous experiment of electric convection, which 

 has recently been repeatedly verified by Fender and Cremieu and 

 others, though deduced from Maxwell's theory, is not incompatible 

 with Weber's view. Again the ratio between the electrostatic and 

 the electromagnetic system of units, repeatedly determined from 

 the early measurement of Maxwell (1868) to the recent elaborate 

 determinations of Abraham (1892) and Margaret Maltby (1897), 

 with an ever closer approach to the velocity of light, was at its incep- 

 tion 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 instantane- 

 ous transmission of electromagnetic force. Maxwell's equations, while 

 equally universal with the preceding, 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 discrimination. 



As a consequence of the resulting enthusiasm, perhaps, there has 

 been but little reference in recent years to the great investigation 

 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, Helm- 

 holtz 's electromagnetic potential is deduced rigorously from funda- 

 mental principles, and contains, as Duhem (1901) showed, the electro- 

 magnetic theory of light. 



Maxwell's own vortex theory of physical lines of force (1861, 

 1862) probably suggested his equations. In recent years, however, 

 the efforts to deduce them directly from apparently simpler proper- 

 ties of a continuous medium, as for instance from its ideal elastics, 

 or again from a specialized ether, have not been infrequent. Kelvin 

 (1890), with his quasi-rigid ether, Boltzmann (1893), Sommerfeld 

 (1892), and others have worked efficiently in this direction. On the 

 other hand, J. J. Thomson (1891, et seq.), with remarkable intuition, 

 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 Poincare* should not be forgotten: If, says 

 Poincare", compatibly with the principle of the conservation of energy 



