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120 burning or oxidation, heat is set free, and the heat is the energy that drives the piston. The piston moves and drives all kinds of machinery, that is to say, the heat that moves the piston, through the medium of the steam, is transformed partly into what the engineer would call mechanical energy. It is this mechanical energy that does work. All the energy, however, set free by oxidation from the fuel is not transformed into mechanical energy. A part, a large part, as much as eighty-eight per cent of it, is set free simply as heat, which, I need hardly say, is of no use to the engineer. The same kind of reasoning guides engineers in the construction of all kinds of engines, and they are always striving to get as much as possible of the energy of the fuel transformed into mechanical energy.

Now turn to our muscle. Is it also a transformer of energy? If it is capable of manifesting mechanical energy, as undoubtedly it is in doing the work of lifting a weight, and if it becomes hot, these two energies, mechanical and thermal, must come from somewhere. Living though it be, it can no