That fire is really a substance, and not a quality, appears from its acting upon other substances, the reality of which has never been doubted. Charcoal, in its natural state, contains within its pores a large quantity of air; but if charcoal is heated, this air is expelled by the fire, which assumes its place, and occupies the pores of the charcoal. The burning of lime also, which deprives it of a great part of its weight by expelling the fixable air, demonstrates that fire, as a substance, enters into the pores of the lime, and forces out those other substances which are least intimately combined with it. Collision or friction of solid bodies, is the means most generally used for exciting the action of fire. The vacuities of all solid bodies are replete with fire, so that it is impossible to agitate or separate their parts swiftly, without giving the same rapid motion to the element contained within them. When a piece of hardened steel is struck with a flint, some particles of the metal are scraped away from the mass, and so violent is the fire which follows the stroke, that it melts and vitrifies them. If the fragments of steel are catched upon paper, and viewed with a microscope, you will find most of them perfect spherules, and very highly polished. Their sphericity demonstrates that they have been in a fluid state, and the polish upon their surface shows them to be vitrified; the fire being disengaged with violence, disposes the particles of the substance to combine with the vital air, while this air accelerates the combustion. The whole of the heat produced is not afforded by the body itself, because in proportion as the interior fire is disengaged, the external air acts upon the body and gives out fire. If the irons at the axis of a coach-wheel are applied to each other, without the interposition of some unctuous matter to keep them from immediate contact, they will become so hot when the carriage runs swiftly along, as to set the wood on fire; and the fore wheels being smallest, and making more revolutions, will be most in danger. The same will happen to mill-work, or any other machinery, if the necessary precautions are neglected. It is no uncommon practice with a blacksmith to use a plate of iron as an extemporaneous tinder-box; for it may be hammered on an anvil till it becomes red hot, and will fire a match of brimstone. A strong man who strikes quick, and keeps turning the iron, so that both sides may be equally exposed to the force of the hammer, will perform this in less time than would be expected. If in the coldest season you lay one dense iron plate upon another, and press the upper one, by a weight, on the lower one, and then rub the one over the other; by reciprocal motions, they will first grow warm, and at length so hot, as in a short time to emit sparks, and at last grow red hot, as if taken out of a vehement fire.