A little reflection, however, ought to convince any one that this conception of the universe--as a machine which is kept in perpetual motion by the reciprocal action of its parts--is a false analogy. And its fallacy is apparent from this, that the moving force of every machine is not inherent in the machine, but some natural primary force distinct from the machine, such as gravity, or the primary atomic forces of attraction and repulsion; and consequently the very idea of mechanism _assumes_ the existence of those primary forces of which it is the professed object of a mechanical theory of the universe to give an explanation. A machine "can no more create energy than it can create matter;" its sole function is "to transform energy into a kind most convenient for us."[257] "We may with the greatest ease convert mechanical work into heat, but we can not by any means convert all the energy of heat back again into mechanical work. In the steam-engine we do what can be done in this way, but it is a very small portion of the whole energy of the heat that is convertible into work, for a large portion is dissipated, and will continue to be dissipated however perfect our engine may become. Let the greatest care be taken in the construction and working of a steam-engine, yet we shall not succeed in converting one fourth of the whole energy of the heat of the coals into mechanical work."[258] It is impossible to construct a machine that can do work without parting with energy; and when the energy is all parted with, any machine whatever must necessarily cease to do any more work unless a fresh supply of energy be brought in from without. It is impossible to make a water-mill work without a constantly renewed supply of water, or to make a steam-engine work without a constantly renewed supply of fuel. "Every one who understands mechanics knows that any such inexhaustible supply of energy is impossible by means of merely mechanical arrangements; but it is equally true, though not perhaps equally so evident, that it is impossible by means of any arrangement of thermal, electric, or chemical forces."[259]