KINETIC OR MECHANICAL VIEW OF NATURE. 75 



the magnetic actions which liclong to all Kubstances ; 

 introduced the term magnetic " permealjilily " ' as de- 

 scriptive of the degree in which various substances 

 acquire magnetic properties and conduct the lines of 

 magnetic force in the neighbourhood of powerful mag- 

 nets ; and finally demonstrated how, if these properties 

 were considered as having different degrees in the dif- 

 ferent axes of crystals, in analogy with the dillerent 

 elasticities which they exhibited, the consequence would 

 be a turning effect which would explain the changed 

 optical properties of crystals under the influence of 



magnetic action. 



Tn these investigations the ideas of 



' Tliis property was afterward.s 

 termed " permeabifity " by Thom- 

 son (Reprint, p. 489, 1872). The 

 general rule of magnetic action can 

 then be expressed by saying that 

 " by virtue of differential action a 

 body may behave paramagnetically 

 or diamagnetically according as it 

 is j)laced in a less or a more perme- 

 able medium than itself " (Clirystal 

 in article " Magnetism," ' Ency. 

 Brit.,' 9th ed., vol. xv. p. 248). 



- "On Ihe Theory of Magnetic 

 Induction in Crystalline and Non- 

 crystalline Substances " ( ' Philos. 

 Mag.,' March 18.57 ; also Reprint, 

 2nd ed., p. 471, <&c.) Poisson had 

 alreadj' foreseen the mathematical 

 possibility of wliat Faraday termed 

 magne- (correctly magneto-) crys- 

 tallic action, but " ce cas singulier 

 ne s'ctant pas encore prdsente h. 

 I'observation, nous I'exclurons de 

 nos recherches " (" Mumoire sur la 

 Thcorie du Magndt.isme," ' Mem. de 

 I'Institut, Paris, 1826,' quoted by 

 Thomson, Reprint, p. 484). .Stimu- 

 lated by the discoveries of Faraday, 

 Pliicker at Bonn, during the extra- 

 ordinary interval wliicli separated 

 the second from tlie first jjcriod of 

 his original geometrical speculations 

 (see vol. i. ]>. 242 of this work), de- 



voted himself to the study of the 

 electric and magnetic jirnjierties of 

 givses and crystals, and in 1847 

 commenced that remarkalile series 

 of physical memoirs through which 

 he became the fellow-worker, if not 

 the rival, of Faradaj-. One of hia 

 first discoveries wa.s the action of 

 magnets on crystals, published in 

 1847 (Pogg. Ann., or Pliicker's 

 ' Physicalische Abhandlungen,' ed. 

 Pockels, Leij)zig, 1896, p. 6, &c.), 

 which supplied to Thomson "the 

 verj' circumstance the observation 

 of which wa.s wanting to induce 

 Poisson to enter ujion a full treat- 

 ment of the subject, and made the 

 working out of a mathematical 

 theory of magnetic induction . . . 

 independently of any hyjwthesis 

 . . . ujion a purely exjierimental 

 foundation . . . imjwrtant " (Tliom- 

 son, loc. cit., p. 471). Pliicker was 

 an original thinker, and mainly a 

 self-taught geniu.-s, imjierfectly ac- 

 quainted with the labours of liis 

 contemi)oraries or predecessors. 

 Thi.s has been noted i>y his biog- 

 raphers as much in liis geometrical 

 as in his physical researches (.see 

 tlic memoirs of Cleb.sch and of Prof. 

 Riecke, jncfixed to the two volumes 

 of the'Gesammclte Abliandlungen '). 



