KINETIC OR MECHANICAL VIEW OF NATURE. 75 



the magnetic actions which belong to all substances ; 

 introduced the term magnetic " permeability " l 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 different 

 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. 2 In these investigations the ideas of 



1 This property was afterwards 

 termed " permeability " 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 placed in a less or a more perme- 

 able medium than itself " (Chrystal 

 in article " Magnetism," ' Ency. 

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



2 "On the Theory of Magnetic 

 Induction in Crystalline and Non- 

 crystalline Substances " ( ' Philos. 

 Mag.,' March 1857 ; also Reprint, 

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

 already foreseen the mathematical 

 possibility of what Faraday termed 

 magne- (correctly magneto-) crys- 

 tallic action, but " ce cas singulier 

 ne s'e'tant pas encore pre'sente' a 

 1'observation, nous 1'exclurons de 

 nos recherches " (" Memoire sur la 

 The"orie du Magne"tisme," ' Mem. de 

 1'Institut, Paris, 1826,' quoted by 

 Thomson, Reprint, p. 484). Stimu- 

 lated by the discoveries of Faraday, 

 Pliicker at Bonn, during the extra- 

 ordinary interval which separated 

 the second from the first period of 

 his original geometrical speculations 

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



voted himself to the study of the 

 electric and magnetic properties of 

 gases and crystals, and in 1847 

 commenced that remarkable series 

 of physical memoirs through which 

 he became the fellow-worker, if not 

 the rival, of Faraday. One of his 

 first discoveries was the action of 

 magnets on crystals, published in 

 1847 (Pogg. Ann., or Pliicker's 

 ' Physicalische Abhandlungen,' ed. 

 Pockels, Leipzig, 1896, p. 6, &c.), 

 which supplied to Thomson "the 

 very circumstance the observation 

 of which was wanting to induce 

 Poisson to enter upon a full treat- 

 ment of the subject, and made the 

 working out of a mathematical 

 theory of magnetic induction . . . 

 independently of any hypothesis 

 . . . upon a purely experimental 

 foundation . . . important " (Thom- 

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

 an original thinker, and mainly a 

 self-taught genius, imperfectly ac- 

 quainted with the labours of his 

 contemporaries or predecessors. 

 This has been noted by his biog- 

 raphers as much in his geometrical 

 as in his physical researches (see 

 the memoirs of Clebsch and of Prof. 

 Riecke, prefixed to the two volumes 

 of the' Gesammelte Abhandlungen'). 



