408 Prof. Plucker on the Electrical Discharge 



mapping) is sought that the difficulty appears, because it is in 

 that part of the problem that the errors in the vertical line have 

 any influence, and in no other part. 

 I am, Gentlemen, 



Your obedient Servant, 

 Calcutta, September 15, 1858. J. H. Pkatt. 



XLVI. Observations on the Electrical Discharge through Rarefied 

 Gases. By M. PlIjcker*. 



[Continued from p. 135.] 

 Magnetic Light, 

 47. IT^HE behaviour towards magnetism of that light which, 

 -i- proceeding from the negative electrode, spreads out 

 in all directions, is so remarkable that I shall in the first place 

 recur to it again. We can best illustrate this behaviour by con- 

 sidering the well-known fact, that when iron filings are strewn 

 upon a piece of stiff paper covering the pole of a magnet, they 

 arrange themselves in curves which have been called magnetic 

 curves, or lines of magnetic force. Such curves render the dis- 

 tribution of the power of a magnet visible even when analysis is 

 unable to determine their form. In every such curve the sepa- 

 rate particles of iron having, under the influence of the magnet, 

 become themselves little magnets, arrange themselves with their 

 attracting poles together so as to form a chain. Could we remove 

 the particles of iron from the influence of gravitation and distri- 

 bute them through the whole space surrounding the magnetic 

 pole, then such chains assuming the form of magnetic curves 

 would traverse the whole magnetic field, and furnish a visible 

 image of the distribution of the magnetic force. The links of 

 every chain would be in a condition of stable equilibrium ; every 

 portion of every chain, if removed by an external force from its 

 position, would, when left to itself, return to its original place 

 again. Never more than one single magnetic curve passes 

 through a given point ; and the position which any one link of 

 the chain occupies in space determines the form of the whole 

 chain; so that if we were to fasten such a link rigidly, and then 

 give the chain any other form, it would, on being again left to 

 itself, reassume the original form of the magnetic curve. Let 

 us imagine the links of a chain, thus withdrawn from the influ- 

 ence of gravity, still to adhere together after having lost their 

 magnetism in consequence of the cessation of the inductive action 



* Translated by Dr. F. Guthrie, from Poggendorff's Annalen, vol. civ. 

 p. 113. 



