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SCIENCE. 



[N. S. Vol. XXII. No. 561. 



or photographic light vector of Fresnel and 

 Cauchy and the magnetic vector of Neu- 

 mann and MacCuUagh. Its predictions 

 have, moreover, been astonishingly verified 

 by the work of Hertz (1890), and it is 

 to-day acquiring added power in the con- 

 vection theories of Lorentz (1895) and 

 others. 



ELECTROSTATICS. 



Coulomb's (1785) law antedates the cen- 

 tury; indeed, it was known to Cavendish 

 (1771, 1781). Problems of electric distri- 

 bution were not seriously approached, how- 

 ever, until Poisson (1811) solved the case 

 for spheres in contact. Afterwards Clau- 

 sius (1852), Helmholtz (1868) and Kirch- 

 hoff (1877) examined the conditions for 

 discs, the last giving the first rigorous 

 theory of the experimentally important 

 plate condenser. In 1845, 1848 the investi- 

 gation of electric distribution received new 

 incentive as' an application of Kelvin's 

 beautiful method of images. Maxwell 

 ('Treatise,' 1873) systematized the treat- 

 ment of capacity and induction coefficients. 



Riess (1837) in a classic series of experi- 

 ments on the heat produced by electrostatic 

 discharge virtually deduced the potential 

 energy of a conductor and in a measure 

 anticipated Joule's law (1841). In 1860 

 appeared Kelvin's great paper on the elec- 

 tromotive force needed to produce a spark. 

 As early as 1855, however, he had shown 

 that the spark discharge is liable to be of 

 the character of a damped vibration and 

 the theory of electric oscillation was subse- 

 quently extended by Kirchhotf (1867). 

 The first adequate experimental verification 

 is due to Feddersen (1858, 1861). 



The specific inductive capacity of a 

 medium with its fundamental bearing on 

 the character of electric force was discov- 

 ered by Faraday in 1837. Of the theories 

 propounded to account for this property 

 the most far reaching is Maxwell's (1865), 



which culminates in the unique result 

 showing that the refraction index of a 

 medium is the square root of its specific 

 inductive capacity. With regard to Max- 

 well's theory of the Faraday stress in the 

 ether as compared with the subsequent 

 development of electrostriction in other 

 media by many authors, notably by Boltz- 

 mann (1880) and by Kirchhoff (1885), it 

 is observable that the tendency of the 

 former to assign concrete physical proper- 

 ties to the tube of force is growing, par- 

 ticularly in connection with radioactivity. 

 Duhem (1892, 1895) insists, however, on 

 the greater trustworthiness of the thermo- 

 dynamic potential. 



The seemingly trivial subject of pyro- 

 electricity interpreted by ^pinus (1756) 

 and studied by Brewster (1825), has none 

 the less elicited much discussion and curi- 

 osity, a vast number of data by Hankel 

 (1839-93) and, others and a succinct ex- 

 planation by Kelvin (1860, 1878). Sim- 

 ilarly piezoelectricity, discovered by the 

 brothers Curie (1880), has been made the 

 subject of a searching investigation by 

 Voigt (1890). Finally Kerr (1875, et 

 seq.) observed the occurrence of double 

 refraction in an electrically polarized me- 

 dium. Recent researches, among which 

 those of Lemoine (1896) are most accurate, 

 have determined the phase difference cor- 

 responding to the Kerr effect under normal 

 conditions, while Voigt (1899) has adduced 

 an adequate theory. 



Certain electrostatic inventions have had 

 a marked bearing on the development of 

 electricity. We may mention in partic- 

 ular Kelvin's quadrant electrometer (1867) 

 and Lippmann's capillary electrometer 

 (1873). Moreover, among apparatus orig- 

 inating in Nicholson's duplicator (1788) 

 and Volta's electrophorus, the Topler- 

 Holtz machine (1865-67), with the re- 

 cent improvement due to Wimshurst, has 



