486 On the Identity of Energy. 
viously pointed out*. The statement is in two parts :— 
(1) Energy cannot be transferred without being transformed ; 
and (2) it always transforms itself from Kinetic to Potential, 
or vice versa. 
When A does work on B energy is transferred from A 
to B; and I say that if the energy which A lost was kinetic, 
then what B gains is potential ; if, on the other hand, A loses 
potential, then B gains kinetic. 
I may make a converse statement, viz. that energy cannot 
be transformed without being transferred; cannot take on a 
different form without being at the same time shifted to a 
different body. So that the common mode of treating a 
falling weight, saying that its energy gradually transforms 
itself from potential to kinetic but remains in the stone all the 
time, is, strictly speaking, nonsense. The fact is the stone 
never had any potential energy, no rigid body can have any; 
the gravitation medium had it however, and kept on transfer- 
ring it to the stone all the time it was descending. 
The above statement, that transformation of energy neces- 
sarily goes on from potential to kinetic or vice versa at every 
act of transfer, almost proves itself. It follows from the fol- 
lowing facts:—When a body possessing potential energy does 
work, its “range ” necessarily diminishes, while the motion 
of the body on which the work is done increases. On the 
other hand, when a moving body does work its motion dimi- 
nishes, and the body which resists the motion, since it yields 
over a certain distance, gains potential energy. Tor the first 
case think of a catapult, bow and arrow, orair-gun. Tor the 
second case think of a bullet fired against a spring and caught 
by it. 
"These examples are favourable and easy, but any others will 
serve equally well to illustrate the matter if regarded from 
the right point of view. Thus a bullet fired upwards gra- 
dually transfers its undissipated energy to the gravitation 
medium, transforming it at the same time into potential. As 
soon as the highest point is reached, the gravitation medium 
proceeds to re-transfer and transform it. A pendulum exhibits 
the alternation of energy from the kinetic to the potential form 
and the accompanying transfer from matter to medium, at 
every half-swing. Any vibrating body does the same; but 
in considering a strained spring we must remember that the 
energy resides not in the spring as a whole but in its elemen- 
tary parts. The strain resides not even in the molecules them- 
selves, perhaps, but in something between those molecules (for 
* Phil. Mag. October 1879, p. 281, sect. 11, and June 1881, p. 582. 
And ‘ Elementary Mechanics,’ § 85. 
