Feb., 1889. 
spencer’s “first principles.” 
35 
repelled by Mr. Spencer’s attempt to reduce both to force. 
They recognise that our muscular sense is misleading inasmuch 
as it gives us a consciousness of loss of energy when we exert 
force alone, and only a consciousness of greater loss joined 
with an inadequate consciousness of motion when we do 
external work. They say that if effort be correlated with 
force it is a mistake to correlate it also with energy, and that 
if we do naturally so correlate it the correlation can only lead 
us astray. 
Another statement which it is difficult to accept, is to the 
effect that the existence which we have termed energy, must 
show itself either as motion or as strain— i.e., either as 
kinetic energy or potential energy. A system in which after 
any interval the kinetic energy comes back when the bodies 
are again in their original position is termed a conservative 
system, and it is of such a system alone that it is strictly 
accurate to say that the sum of the potential and kinetic 
energies remains the same. When and only when we have 
such a system are the forces persistent, i.e., dependent only on 
the distances of the bodies apart. It is supposed (not, as 
Spencer says, assumed ) that astronomy furnishes us with a 
grand example of a conservative system, inasmuch as our 
proofs of the indestructibility of matter lead us to suppose 
that the planets have constant masses, and our measures of 
their distances and motions show that when the distances 
repeat themselves the velocities recur. The masses being 
constant the energy must have all returned. But even in 
this case it is suspected that the forces are not quite persistent, 
though we have no certain proof of the fact.* 
Terrestrial motions are all affected by friction, a sworn 
enemy to conservation, since by opposing the motions it 
always ends them without putting any potential energy in 
their place. Careful examination of cases of friction shows, 
however, that there is still a sign of the continuity of existence 
of that which for a time appeared as kinetic energy, and then 
on vanishing, led us to believe that it still existed as potential 
or strain energy. This hew sign is heat—something affecting 
a new sense. Further study shows other signs—as light 
affecting the sense of sight—and chemical energy, sometimes 
perhaps affecting the sense of taste. Then in some cases the 
phenomena of magnetism and electricity are developed, pheno¬ 
mena which lead us to believe that there is latent energy 
* I may here point out an error into which Spencer appears to have 
fallen, confounding the equality of action and reaction with persistence 
of force. One is a relation true at any instant, the other a relation 
true in successive instants. 
