fluctuation to an unknown extent; nor can we have any, the smallest, right to assert that what is expended in one form _is_ necessarily laid up for further use in the other. It would be very difficult, I apprehend, to show whether, in the winding up of a clock or the building of a pyramid, taking into consideration all the various modes in which _vis viva_ disappears and reappears in the expenditure of muscular power, the evolution of animal heat, the consumption of the materials of our tissues, the propagation of vibratory motions, and a thousand other modes of transfer, the total _vis viva_ of this our planet is increased or diminished. That it should remain absolutely unchanged during the process is in the last degree inconceivable. The amount of _vis viva_ latent in the form of heat or molecular motion in the sun and planets in our immediate system may bear, and probably does bear, a by no means inappreciable ratio to that more distinctly patent in the form of bodily motion in the periodic circulation of the planets round the sun, and the sun and planets round their axes. The latter amount fluctuates to and fro according to laws easily calculable, _but the former we have no means whatever of computing, and to what extent, or within what limits, it may be variable, we are altogether ignorant_."[261] The two dynamical laws of Conservation of Energy and Transformation of Energy can not therefore be regarded as universal and absolute laws; they are particular and derivative laws subject to limitations which are supplied by the third dynamical law--the Dissipation of Energy. The law of the conservation of energy simply asserts "that the whole amount of energy in the universe, or in any limited system which does not receive energy from without, or part with it to external matter, is invariable;" in other words, that every material system subject to no other forces than actions and reactions between its parts is a dynamically conservative system. But Sir William Thomson has shown that "in nature this hypothetical condition is apparently violated in all circumstances of motion. A material system can never be brought through any returning cycle of motion without spending more work against the mutual forces of its parts than is gained from these forces, because no relative motion can take place without meeting with frictional or other forms of resistance."[262] "There can be but one ultimate result for such a system as that of the sun and planets, if continuing long enough under existing laws, and not disturbed by meeting with other moving masses in space. That result is the falling together of all into one mass, which, although rotating for a time, _must in the end come to rest relatively to the surrounding medium_."[263]