the Lines of Magnetic Force. 405 



bable that matter is always essentially present ; but the hypo- 

 thetical sether may perhaps be admitted here as well as elsewhere. 

 No condition of quality or polarity has as yet been recognised 

 in them. In respect of time, it has been found, in the case of a 

 Leyden discharge, that it is necessary even with the best con- 

 ductors ; indeed there is reason to think it is as necessary there 

 as in the cases dependent on bad conducting media, as, for in- 

 stance, in the lightning flash. 



3251. Three great distinctions at least may be taken among 

 these cases of the exertion of force at a distance; that of gravita- 

 tion, where propagation of the force by physical lines through the 

 intermediate space is not supposed to exist ; that of radiation, 

 where the propagation does exist, and where the propagating line 

 or ray, once produced, has existence independent eitherof its source 

 or termination ; and that of electricity, where the propagating 

 process has intermediate existence, like a ray, but at the same time 

 depends upon both extremities of the line of force, or upon condi- 

 tions (as in the connected voltaic pile) equivalent to such extre- 

 mities. Magnetic action at a distance has to be compared with 

 these. It may be unlike any of them ; for who shall say we are 

 aware of all the physical methods or forms under which force is 

 communicated ? It has been assumed, however, by some, to be 

 a pure case of force at a distance, and so like that of gravity ; 

 whilst others have considered it as better represented by the idea 

 of streams of power. The question at present appears to be, 

 whether the lines of magnetic force have or have not a physical 

 existence; and if they have, whether such physical existence has 

 a static or dynamic form (3075. 3156. 3172. 3173.). 



3252. The lines of magnetic force have not as yet been affected 

 in their qualities, i. e. uothing analogous to the polarization of a 

 ray of light or heat has been impressed on them. A relation 

 between them and the rays of light when polarized has been 

 discovered (2146.)*; but it is not of such a nature as to give 

 proof as yet, either that the lines of magnetic force have a sepa- 

 rate existence, or that they have not ; though I think the facts 

 are in favour of the former supposition. The investigation is an 

 open one, and very important. 



3253. No relation of time to the lines of magnetic force has 

 as yet been discovered. That iron requires time for its magneti- 

 zation is well known. Pliicker says the same is the ease for 

 bismuth, but I have not been able to obtain the effect showing 

 this result. If that were the case, then mere space with its 

 ether ought to have a similar relation, for it comes between bis- 

 muth and iron (2787.) ; and such a result would go far to show 

 that the lines of magnetic force had a separate physical existence. 



* Philosophical Transactions, 1846, p. 1. 



