206 Dr. Tyndall on the Existence of a 



netic pole. Substituting elongated tubes for spheres, you find 

 that when a tube containing a solution of a certain strength is 

 suspended in a weaker solution, between the two poles of a 

 magnet, the tube sets from pole to pole; but that when the 

 solution tuithout the tube is stronger than that within it, the 

 tube recedes from the pole and sets equatorially. 



Here then, you state, are the phsenomena of diamagnetism. 

 It is maintained by some, that, to account for these phseno- 

 mena, it is necessary to assume, in the case of diamagnetic 

 bodies, the existence of a polarity the reverse of that of iron. 

 But nobody will affirm that the mere fact of its being suspended 

 in a stronger solution reverses the polarity of a magnetic liquid : 

 — to account for the repulsion of the weak solution, when sub- 

 mersed in a stronger one, no such hypothesis is needed ; why 

 then should it be thought necessai'y in the case of so-called dia- 

 magnetic bodies ? It is only by denying that space presents a 

 medium which bears the same relation to diamagnetic bodies 

 that the stronger magnetic solution bears to the weaker one, 

 that the hypothesis of a distinct diamagnetic polarity is at all 

 rendered necessary. 



The effects upon which the foregoing striking argument is 

 based are differential ones, and are embraced, as already observed 

 by M. E. Becquerel, by the so-called principle of Archimedes. 

 This principle, in reference to the case before us, affirms that the 

 body immersed in the liquid is attracted by a force equal to the 

 difference of the attractions exerted upon the liquid and the body 

 immersed in it. Hence, if the attraction of the liquid be less 

 than that of the immersed body, the latter will approach the 

 pole ; if the former attraction be the greater, the immersed body 

 recedes from the pole, and is apparently repealled. The action 

 is the same as that of gravity upon a body immersed in water : 

 if the body be more forcibly attracted, bulk for bulk, than the 

 water, it sinks ; if less forcibly attracted, it rises ; the mechanical 

 effect being the same as if it were repelled by the earth. 



The question then is, are all magnetic phsenomena the result 

 of a differential action of this kind ? Does space present a 

 medium less strongly attracted than soft iron, and more strongly 

 attracted than bismuth, thus permitting of the approach of the 

 former, but causing the latter to recede from the pole of a 

 magnet ? If such a medium exists, then diamagnetism, as you 

 incline to believe, merges into ordinary magnetism, and " the 

 polarity of the magnetic force/' in iron and in bismuth, is one 

 and the same. 



Pondering upon this subject a few evenings ago, and almost 

 despairing of seeing it ever brought to an experimental test, a 

 thought occurred to me, which, when it first presented itself, 



