"Now we see, uncle," Harry continued, "that water is hydrogen and oxygen united together, that water is got wherever hydrogen is burnt in common air, that a candle won't burn without air, and that when a candle burns there is hydrogen in it burning, and forming water. Now, then, where does the hydrogen of the candle get the oxygen from, to turn into water with it?" "From the air, eh?" "Just so. I can't stop to tell you of the other things which there is oxygen in, and the many beautiful and amusing ways of getting it. But as there is oxygen in the air, and as oxygen makes things burn at such a rate, perhaps you wonder why air does not make things burn as fast as oxygen. The reason is, that there is something else in the air that mixes with the oxygen and weakens it." "Makes a sort of gaseous grog of it, eh?" said Mr. Bagges. "But how is that proved?" "Why, there is a gas, called nitrous gas, which, if you mix it with oxygen, takes all the oxygen into itself, and the mixture of the nitrous gas and oxygen, if you put water with it, goes into the water. Mix nitrous gas and air together in a jar over water, and the nitrous gas takes away the oxygen, and then the water sucks up the mixed oxygen and nitrous gas, and that part of the air which weakens the oxygen is left behind. Burning phosphorus in confined air will also take all the oxygen from it, and there are other ways of doing the same thing. The portion of air left behind is called nitrogen. You wouldn't know it from common air by the look; it has no color, taste, nor smell, and it won't burn. But things won't burn in it either; and any thing on fire put into it goes out directly. It isn't fit to breathe; and a mouse, or any animal, shut up in it dies. It isn't poisonous, though; creatures only die in it for want of oxygen. We breathe it with oxygen, and then it does no harm, but good; for if we breathe pure oxygen, we should breathe away so violently, that we should soon breathe our life out. In the same way, if the air were nothing but oxygen, a candle would not last above a minute." "What a tallow-chandler's bill we should have!" remarked Mrs. Wilkinson. "'If a house were on fire in oxygen,' as Professor Faraday said, 'every iron bar, or rafter, or pillar, every nail and iron tool, and the fire-place itself; all the zinc and copper roofs, and leaden coverings, and gutters, and; pipes, would consume and burn, increasing the combustion.'" "That would be, indeed, burning 'like a house on fire,'" observed Mr. Bagges. "'Think,'" said Harry, continuing his quotation, "'of the Houses of Parliament, or a steam-engine manufactory. Think of an iron-proof chest--no proof against oxygen. Think of a locomotive and its train--every engine, every carriage, and even every rail would be set on fire and burnt up.' So now, uncle, I think you see what the use of nitrogen is, and especially how it prevents a candle from burning out too fast."