September 29, 1905.] 



SCIENCE. 



391 



Poisson (1824-27). The interpretation 

 thus given to the mechanism of two condi- 

 tionally separable magnetic fluids facili- 

 tated discussion and was very generally 

 used in argument, as for instance by Gauss 

 (1833) and others, although Ampere had 

 suggested the permanent molecular current 

 as early as 1820. Weber (1852) intro- 

 duced the revolable molecular magnet, a 

 theory which Ewing (1890) afterwards 

 generalized in a way to include magnetic 

 hysteresis. The phenomenon itself was 

 independently discovered by Warburg 

 (1881) and by Ewing (1882) and has since 

 become of special practical importance. 



Faraday in 1852 introduced his inval- 

 uable conception of lines of magnetic force, 

 a geometric embodiment of Gauss's (1813, 

 1839) theorem of force flux, and Maxwell 

 (1855, 1862, et seq.) thereafter gave the 

 rigorous scientific meaning to this concep- 

 tion, which pervades the whole of cotem- 

 poraneous electromagnetics. 



The phenomenon of magnetic induction, 

 treated hypothetically by Poisson (1824r- 

 27) and even by Barlow (1820), has since 

 been attacked by many great thinkers, like 

 F. Neumann (1848), Kirchhoff (1854); 

 but the predominating and most highly 

 elaborated theory is due to Kelvin (1849, 

 et seq.). This theory is broad enough to 

 be applicable to seolotropic media and to it 

 the greater part of the notation in current 

 use throughout the world is due. A new 

 method of attack of great promise has, 

 however, been introduced by Duhem (1888, 

 1895, et seq.) in his application of the 

 thermodynamic potential to magnetic phe- 

 nomena. 



Magnetieians have succeeded in express- 

 ing the magnetic distribution induced in 

 certain simple geometrical figures like the 

 sphere, the spherical shell, the ellipsoid, 

 the infinite cylinder, the ring. Green in 

 1828 gave an original but untrustAvorthy 

 treatment for the finite cylinder. Lamel- 



lar and solenoidal distributions are defined 

 by Kelvin (1850), to whom the similarity 

 theorems (1856) are also due. Kirchhoff's 

 results for the ring were practically utilized 

 in the absolute measurements of Stoletow 

 (1872) and of Rowland (1878). 



Dimagnetism, though known since Brug- 

 mans (1778), first challenged the perman- 

 ent interest of science in the researches of 

 Becquerel (1827) and of Faraday (1845). 

 It is naturally included harmoniously in 

 Kelvin's great theory (1847, et seq.). In- 

 dependent explanations of diamagnetism, 

 however, have by no means abandoned the 

 field; one may instance Weber's (1852) in- 

 genious generalization of Ampere's molec- 

 ular currents (1820) and the broad critical 

 deductions of Duhem (1889) from the 

 thermodynamic potential. For the treat- 

 ment of a^olotropic magnetic media, Kel- 

 vin's (1850, 1851) theory seems to be 

 peculiarly applicable. Weber's theory 

 would seem to lend itself well to electronic 

 treatment. 



The extremely complicated subject of 

 magnetostriction, originally observed by 

 Matteuci (1847) and by Joule (1849) in 

 different cases, and elaborately studied by 

 Wiedemann (1858, et seq.), has been re- 

 peatedly attacked by theoretical physicists, 

 among whom Helmholtz (1881), Kirchhoff 

 (1885), Boltzmann (1879) and Duhem 

 (1891) may be mentioned. None of the 

 carefully elaborated theories accounts in 

 detail for the facts observed. 



The relations of magnetism to light have 

 increased in importance since the funda- 

 mental discoveries of Faraday (1845) and 

 of Verdet (1854), and they have been 

 specially enriched by the magneto-optic 

 discoveries of Kerr (1876, et seq.), of 

 Kundt (1884, et seq.), and more recently 

 by the Zeemann effect (1897, et seq.). 

 Among the theories put forth for the lat- 

 ter, the electronic explanation of Lorentz 

 (1898, 1899) and that of Voigt (1899) are 



