All this appears to have been clearly perceived by Newton, who gave it as a _second_ interpretation of his Third Law of Motion. His statement is equivalent, in modern language, to the following:[36]—_Work done on any system of bodies has its equivalent in the form of work done against friction, molecular forces, or gravity, if there be no acceleration; but if there be acceleration, part of the work is expended in overcoming resistance to acceleration, and the additional kinetic energy developed is equivalent to the work so spent._ 100. Thus Newton expressly tells us (though not in these words) that we are to include in the same category work done by or against a force—whether that force be due to gravity, friction, or molecular action (such as elasticity, for instance), or even to acceleration. (_a._) When work is done against gravity, as in lifting a mass from the ground, we have just seen that it is (as it were) stored up in the raised mass; we can recover it at any time by letting the mass descend. Thus it is that we furnish a clock with motive power sufficient to keep it going for a week in spite of friction and other resistance, by simply winding up its weights. (_b._) When work is done against molecular forces, we have a similar storing up, as, for instance, in drawing a bow or in winding up a watch. (_c._) When work is done against the inertia of a body, _i.e._ to accelerate its velocity, Newton’s definitions show that the additional kinetic energy so produced is equal to the work so spent. (_d._) In abstract dynamics we simply consider as lost the work spent against friction. In Newton’s time it was not known what became of it. 101. Leaving out, then, for the present, the fourth alternative, we see that whatever work is spent, we must, according to Newton, even in abstract dynamics recognise that _it is not lost, but only transformed_ into an equivalent quantity stored up for future use, either in a quiescent form (as, for instance, the potential energy of a raised weight or bent spring), or in an active form (as the kinetic energy of a moving mass). Here, then, at last, we recognise the same sort of conservation as that which we found in matter. But the statement so far is defective, as we have seen, in one particular. What becomes of work spent in overcoming friction? or what becomes of the energy of the blacksmith’s hammer after it has struck the anvil? To this experiment alone can give the answer. Let us see what it has told us.