edison 
LIV. On the Identity of Energy: in connection with Mr. 
Poynting’s Paper on the Transfer of Energy in an Electro- 
magnetic Field; and on the two Fundamental Forms of 
Energy. By Ourver Lover, D.Sc.* 
[‘ is well known that Prof. Poynting has communicated 
to the Royal Society a most admirable and important 
paper, “ On the Transfer of Hnergy in the Hlectromagnetic 
Field”? t; a paper which cannot but exert a distinct in- 
fluence on all future writings treating of electric currents. 
In that paper he introduces the idea of continuity in the 
existence of energy—a natural though not a necessary conse- 
quence of its conservation; so that, whenever energy is trans- 
ferred from one place to another at a distance, it is not to be 
regarded as destroyed at one place and recreated at another, 
but it is to be regarded as transferred, just as so much matter 
would have to be transferred; and accordingly we may seek 
for it in the intervening space, and may study the paths by 
which it travels. 
This notion is, I say, an extension of the principle of the 
conservation of energy. The conservation of energy was 
satisfied by the total quantity remaining unaltered; there 
was no individuality about it: one form might die out, 
provided another form simultaneously appeared elsewhere in 
equal quantity. On the new plan we may label a bit of 
energy and trace its motion and change of form, just as we 
may ticket a piece of matter so as to identify it in other 
places under other conditions ; and the route of the energy 
may be discussed with the same certainty that its existence 
was continuous as would be felt in discussing the route of 
some lost luggage which has turned up at a distant station in 
however battered and transformed a condition. 
In this new form the doctrine of the conservation of energy 
is really much simpler and more satisfactory than in its old 
form ; and the doctrine may be proved rigidly and instanta- 
neously from two very simple premises, viz. Newton’s law 
of motion on the one hand, and the denial of action at a 
distance on the other; as I endeavoured in this Magazine some 
time ago to showf, and will now repeat. 
I speak of Newton’s law of motion because I believe it will 
be admitted that Newton’s three laws of motion, in so far as 
* Communicated by the Author, 
+ Poynting, Phil. Trans. ii. 1884, p. 348, 
{ Phil. Mag. January 1881, p. 36; and June 1881, p. 531. Also 
‘Elementary Mechanics’ (Chambers), § 80. 
