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since Newton’s days. But its application was limited by the 
opinion, common to Newton and many others, that the atoms, 
being finite and hard, might collide with each other. In this 
case motion would be destroyed when they met with opposite 
velocities, and the formula would fail. But all later researches 
have rendered this hypothesis of atomic collision less and less 
probable, though they can hardly be said to have proved its 
falsehood. And thus the Conservation of Vis viva , from a 
mere conception or hypothesis, has risen into the dignity of a 
probable fact, so far as physical forces are concerned, in the 
actual constitution of the material universe ; and some analysts 
have coined for it this new name, the Conservation of 
Energy. 
This old formula of dynamics, borrowed from mathematicians, 
has passed into the hands of experimentalists in physics and 
physiology. It then becomes the Indestructibility of Force, 
and is announced as a grand scientific discovery of the last 
thirty years. Mr. Spencer, the great apostle of evolution, goes 
a step further. He calls it the Persistence of Force, and affirms 
it to be no result of experience, but an ultimate, self-evident 
truth, of which no inductive proof is possible. Its denial is a 
pseud-idea, and unthinkable. The human mind, he says, is 
incapable of thinking the opposite. It is a truth “ defying 
contradiction, and transcending demonstration.” Even this 
does not exhaust its claim on our faith : it is “ the sole truth 
which transcends experience.” 
But let us descend from this lofty cloudland, this extreme 
dogmatism of a wholly sceptical philosophy, to the humbler 
region of plain reason and common sense. Before we can 
decide the controversy whether this doctrine is true or false, a 
great recent discovery, or a greater a priori truth, which men 
have always held and could not help holding, because its oppo- 
site is unthinkable, or itself a demonstrable falsehood, the mere 
product of confused thought, we must first settle what it 
really means. Is it Force or Energy of which it speaks ? Or 
are Force and Energy the same ? If distinct, is the doctrine 
true of both, or of either ? Is the indestructibility by human 
power only, or by any power, human or divine ? Is it a con- 
servation without any preserver, or a persistence without any 
person or thing that persists and perseveres? Is it indestruc- 
tibility when no one attempts to destroy, and when there is 
no existence, nothing but an abstract quality, or the mere 
total of an arithmetical reckoning, to be destroyed ? Let us 
try to unravel this tangled skein, so that we may see clearly 
the true character of this great experimental discovery, or still 
YOL. IX. X 
