158 Prof. Tyndall on the Nature of the Force by which 



The foregoing introductory statement will, perhaps, sufficiently 

 indicate the present aspect of this question. The object I pro- 

 posed to myself in commencing the inquiry now laid before the 

 Royal Society, was to obtain, if possible, clearer notions of the 

 nature of the diamagnetic force than those now prevalent ; for 

 though, in the preceding paragraphs, we have touched upon 

 some of the most complex phsenomena of magnetism and dia- 

 magnetism, and are able to produce these phsenomena at will, 

 the greatest diversity of opinion still prevails as to the real re- 

 lationship of the two forces. The magnetic force, we know, 

 embraces both attraction and repulsion, thus exhibiting that 

 wonderful dual action which we are accustomed to denote by 

 the term polarity. Mr. Faraday was the first who proposed the 

 hypothesis that diamagnetic bodies, operated on by magnetic 

 forces, possess a polarity " the same in kind as, but the reverse 

 in direction, of that acquired by iron, nickel, and ordinaiy mag- 

 netic bodies under the same circumstances*." M. W. Weber 

 sought to confirm this hypothesis by a series of experiments, 

 wherein the excitement of the supposed diamagnetic polarity 

 was applied to the generation of induced currents — appai'ently 

 with perfect success. Mr. Faraday afterwards showed, and his 

 results were confirmed by M. Verdet, that eff'ects similar to those 

 described by the distinguished German, were to be attributed, 

 not to the excitement of diamagnetic polarity, but to the genera- 

 tion of ordinary induced cui-rents in the metallic mass. On the 

 question of polarity IMr. Faraday's results were negative, and he 

 therefore, with philosophic caution, holds himself unpledged 

 to his early opinion. M. Weber, however, still retains his belief 

 in the reverse polarity of diamagnetic bodies, whereas Weber's 

 countryman M. von Feilitsch, in a series of memoirs recently 

 published in Poggendorfi"'s Annalen, contends that the polarity 

 of diamagnetic bodies is precisely the same as that of magnetic 

 ones. In this unsettled state of the question nothing remained 

 for me but a complete examination of the nature of the dia- 

 magnetic force, and a thorough comparison of its phaenomena 

 with those of ordinary magnetism. This has been attempted 

 in the following pages : with what success it must be left to the 

 reader to decide. 



Before entering upon the principal inquiry, I will introduce 

 one or two points which arose incidentally from the investiga- 

 tion, and which appear to be worth recording. 



I. On the Magnetic Properties of Wood. 



No experiments have yet been made, to determine the influence 

 of structure upon the magnetic deportment of this substance ; 



* Experimental Researches, 2429, 2430. 



