218 NOTICES OF THE MEETINGS [June 11, 
these two conditions must be exactly equal in amount, not merely to 
produce the effects of attraction, but in every other case ; for it is im- 
possible so to arrange things that there shall be present or be evolved 
more electric power of the one kind than of the other. Another 
limitation is that they must be in physical relation to each other ; 
and that when a positive and a negative electrified surface are thus 
associated, we cannot cut off this relation except by transferring the 
forces of these surfaces to equal amounts of the contrary forces 
provided elsewhere. Another limitation is that the power is defi- 
nite in amount. Ifa balla be charged with 10 of positive electricity 
it may be made to act with that amount of power on another ball 6 
charged with 10 of negative electricity; but if 5 of its power be 
taken up by a third ball ¢ charged with negative electricity, then it 
can only act with 5 of power on ball a, and that ball must find or 
evolve 5 of positive power elsewhere: this is quite unlike what 
occurs with gravity, a power that presents us with nothing dual in 
its character. Finally the electric force acts in curved lines. Ifa 
ball be electrified positively and insulated in the air,and a round metal- 
lic plate be placed about 12 or 15 inches off, facing it and uninsulated, 
the latter will be found, by the necessity mentioned above, in a 
negative condition; but it is not negative only on the side facing 
the ball but on the other or outer face also, as may be shewn by a carrier 
applied there, or by a strip of gold or silver leaf hung against that 
outer face. Now the power affecting this face does not pass 
through the uninsulated plate, for the thinnest gold leaf is able to 
stop the inductive action, but round the edges of the face and 
therefore acts in curved lines. All these points indicate the ex- 
istence of physical lines of electric force: —the absolutely essential 
relation of positive and negative surfaces to each other, and their 
dependence on each other contrasted with the known mobility 
of the forces, admit of no other conclusion. The action also in curved 
lines must depend upon a physical line of force. And there is a 
third important character of the force leading to the same result, 
namely, its affection by media having different specific inductive 
capacities. 
When we pass to Dynamic Electricity the evidence of physical 
lines of force is far more patent. A voltaic battery, having its ex- 
tremities connected by a conducting medium, has what has been 
expressively called a current of force running round the circuit, but 
this current is an axis of power having equal and contrary forces in 
opposite directions. It consists of lines of force which are com- 
pressed or expanded according to the transverse action of the con- 
ductor, which changes in direction with the form of the conductor, 
which are found in every part of the conductor, and can be taken 
out from any place by channels properly appointed for the purpose ; 
and nobody doubts that they are physical lines of force. 
Finally as regards a Magnet, which is the object of the present 
discourse. A magnet presents a system of forces perfect in itself, 
