ON THE LINES OF MAGNETIC FORCE. 
55 
bodies as water, oil, resin, &c., will probably be included in the same state ; for the 
non-conducting condition, which prevents the formation of a current in them, does 
not militate against the existence of that condition which is prior to the effect of 
motion. A piece of copper, which cannot have the current, because it is not in a 
circuit (308/.), and a piece of lac, which cannot, because it is a non-conductor of 
electricity, may have peculiar but analogous states when moving across a field of 
magnetic power. 
3174. On bringing this paper to a close, I cannot refrain from again expressing 
my conviction of the truthfulness of the representation, which the idea of lines of 
force affords in regard to magnetic action. All the points which are experimentally 
established with regard to that action, i. e. all that is not hypothetical, appear to be 
well and truly represented by it. Whatever idea we employ to represent the power, 
ought ultimately to include electric forces, for the two are so related that one ex- 
pression ought to serve for both. In this respect, the idea of lines of force appears 
to me to have advantages over the method of representing magnetic forces by centres 
of action. In a straight wire, for instance, carrying an electric current, it is appa- 
rently impossible to represent the magnetic forces by centres of action, whereas the 
lines of force simply and truly represent them. The study of these lines has, at 
different titnes, been greatly influential in leading me to various results, which I 
think prove their utility as well as fertility. Thus, the law of magneto-electric in- 
duction (114.); the earth’s inductive action (149. 161. 171-) ? ^^e relation of mag- 
netism and light (2146. and note); diamagnetic action and its law (2243.), and 
rnagnecrystallic action (2454.), are cases of this kind : and a similar influence of 
them, over my mind, will be seen in the further instances of the polarity of diamag- 
netic bodies (2640.); the relation of magnetic curves and the evolved electric currents 
(243.); the explication of Arago’s phenomenon (81.), and the distinction between that 
and ordinary magnetism (243.245.); the relation of electric and magnetic forces 
(1709.); the views regarding magnetic conduction (2797 -)j atmospheric mag- 
netism (2847.) I have been so accustomed, indeed, to employ them, and especially 
in my last Researches, that I may, unwittingly, have become prejudiced in their 
favour, and ceased to be a clear-sighted judge. Still, I have always endeavoured to 
make experiment the test and controler of theory and opinion ; but neither by that 
nor by close cross examination in principle, have I been made aware of any error 
involved in their use. 
3175. Whilst writing this paper, I perceive, that, in the late Series of these Re- 
searches, Nos. XXV. XXVI. XXVII., I have sometimes used the term lines offeree 
so vaguely, as to leave the reader doubtful whether I intended it as a merely repre- 
sentative idea of the forces, or as the description of the path along which the power 
was continuously exerted. What I have said in the beginning of this paper (3075.) 
will render that matter clear. I have as yet found no reason to wish any part of 
those papers altered, except these doubtful expressions: but that will be rectified if 
