Electric Energy in Voltaic Circuits. 489 
rodibility of the metals, but involving some other more purely 
physical force also:—much as Bouty surmised from his expe- 
riments on the generation of heat at a metal-liquid junction 
when a current was driven across it. 
A number of tables of the order of metals in different solu- 
tions of different strengths and at different temperatures are 
given, together with some numerical values. 
It is possible that by an attentive study of Dr. Gore’s work, 
and of other work in the same direction, a more com- 
plete account could be given of the exact relation existing 
between what may be called the thermal and the chemical 
contact-forces at metal-liquid junctions ; if indeed any valid 
distinction can actually be drawn between them. 
At present, at any rate, I do not attempt this; but proceed 
to the other matter spoken of, viz. the illustration of my views 
on the voltaic circuit by a diagram of its energy-paths. 
Prof. Poynting has taught us, in a paper of the very greatest 
interest and power”, that modern views of vis a tergo, a3 
opposed to action at a distance, lead necessarily to the idea of 
continuity in the existence of electric, as well as of all other, 
energy, and hence toa study of the paths along which it moves 
from one part of the field to another. He has shown that these 
paths are the intersections of the magnetic and the electro- 
static equipotential surfaces, and that the rate of flow of energy 
is proportional to the product of the electrostatic and electro- 
magnetic intensities at every point. 
This view of the electromagnetic field, though no doubt 
implicitly involved in Maxwell’s equations, yet urgently 
needed to be dragged forth and exhibited ; and it is difficult 
to imagine how the developing process could have been done 
better than it is done in Prof. Poynting’s memoir. 
The special cases by which his memoir is illustrated are not, 
however, in exact accordance with the views of the electric 
circuit which I am advocating; and in particular the diagram 
of the curves for a voltaic current is, in my view, wrong. I 
cannot say I thought it wrong at the first glance; I was 
pleased to see that Mr. Poynting held what I consider the 
correct view, that the main seat of the E.M.F. in a simple pile 
is at the zinc-acid junction and not at the zinc-copper, and 
that his diagram embodied this view. 
But when I came to draw the curves myself for this case, 
taking account of the step of potential which I believe to exist 
between a metal and the air, I found the energy-paths refuse 
to start spreading out from the zinc-acid junction, as we had 
* Poynting, Phil. Trans. 1884, pt. ii, “On the Transfer of Energy in 
the Electromagnetic Field.” 
