produces mechanical energy, and it also becomes hot. But we know that energy never comes out of nothing. You cannot get it for nothing. You can only get it by the expenditure, or, if you put it in another form, by the disappearance, of another mode of energy. This is one of the greatest thoughts of modern times—this thought of the persistence of energy. We cannot create or destroy matter. We can only transform it. In like manner, we cannot create or destroy energy. We can only transform it. Our steam-engines, and gas-engines, and hot-air engines are all transformers of energy. None of them makes it; they receive it from something, and they pass it on in other forms.
Take a steam-engine. It works by the steam expanding by heat and moving the piston. The hot steam comes from the boiler containing the water. The water is heated by the fire of the furnace. In the fire combustion or burning is taking place. The fuel, consisting chiefly of matters rich in carbon, is burnt, that is to say, the carbon unites with the oxygen of the air to form carbonic acid gas; but in this chemical operation which we call
