Then you let bubbles of the gas up into the jar, and they turn out the water and take its place. Put a stopper in the neck of the jar, or hold a glass plate against the mouth of it, and you can take it out of the water and so have bottled oxygen. A lighted candle put into a jar of oxygen blazes up directly, and is consumed before you can say Jack Robinson Charcoal burns away in it as fast, with beautiful bright sparks-phosphorus with a light that dazzles you to look at and a piece of iron or steel just made redhot at the end first, is burnt in oxygen quicker than a stick would be in common air. The experiment of burning things in oxygen beats any fire-works."

"Oh how, jolly !" exclaimed Tom.

"Now we see, uncle," Harry continued, "that water is hydrogen and oxygen united together, that water is got wherever hydroyen 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 oxgyen 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

Mix nitrous gas and air

with it, goes into the water. 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 the air left behind is called nitrogen. You wouldn't know it from common air by the look; it has no colour, taste, nor smell, and it won't burn. But things won't burn in it, either; and anything 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 breathed 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."

“Eh?” said Mr. Bagges.

"Well, I will say I do think we are under considerable obligations to nitrogen."

“I have explained to you, uncle," pursued Harry, “how a candle, in burning, turns into water. But it turns into something else besides that; there is a stream of hot air going up from it that won't condense in dew; some of that is the nitrogen of the air which the candle has taken all the oxygen from. But there is more in it than nitrogen. Hold a long glass tube over a candle, so that the stream of hot air from it may go up through the tube. Hold a jar over the end of the tube to collect some of the hot air. Put some lime-water, which looks quite clear, into the jar; stop the jar, and shake it up. The lime-water, which was quite clear before, turns milky. Then there is something made by the burning of the candle that changes the colour of the lime-water. That is a gas, too, and you can collect it, and examine it. It is to be got from several things, and is a part of all chalk, marble, and the shells of eggs or of shell-fish. The easiest way to make it is by pouring muriatic or sulphuric acid on chalk or marble. The marble or chalk begins to hiss or bubble, and you can collect the bubbles in the same way that you can oxygen. The gas made by the candle in burning, and which also is got out of the chalk and marble, is called carbonic acid. It puts out a light in a moment; it kills any animal that breathes it, and it is really poisonous to breathe, because it destroys life even when mixed with a pretty large quantity of common air. The bubbles made by beer when it ferments, are carbonic acid, so is the air that fizzes out of soda-water, and it is good to swallow though it is deadly

to breathe. It is got from chalk by burning the chalk as well as by putting acid to it, and burning the carbonic acid out of chalk makes the chalk lime. This is why people are killed sometimes by getting in the way of the wind that blows from lime-kilns."

“Of which it is advisable carefully to keep to the windward," Mr. Wilkinson observed.

"The most curious thing about carbonic acid gas," proceeded Harry, "is its weight. Although it is only a sort of air, it is so heavy that you can pour it from one vessel into another. You may dip a cup of it and pour it down a candle, and it will put the candle out, which would astonish an ignorant person; because carbonic acid gas is as invisible as the air, and the candle seems to be put out by nothing. A soap-bubble or common air floats on it like wood on water. Its weight is what makes it collect in brewers' vats; and also in wells, where it is produced naturally; and owing to its collecting in such places it causes the deaths we so often hear about of those who go down in them without proper care. It is found in many springs of water, more or less; and comes out of the earth in some places. is what stupefies the dogs in the Crotto del Cane. Well, but how is carbonic acid gas made by the candle ?"

a great deal of it Carbonic acid gas

"I hope with your candle you'll throw some light upon the subject," said Uncle Bagges.

"I hope so," said Harry. "Recollect it is the burning of the smoke, or soot, or carbon of the candle, that makes the candle-flame bright. Also that the candle won't burn without air. Likewise that it will not burn in nitrogen, or air that has been deprived of oxygen. So the carbon of the candle mingles with oxygen, in burning, to make carbonic acid gas, just as the hydrogen does to form water.

Carbonic acid gas, then, is carbon or charcoal dissolved in oxygen. Here is black soot getting invisible and changing into air; and this seems strange, uncle, doesn't it?"

"Ahem!

Strange, if true," answered Mr. Bagges. "Eh?-well! I suppose it's all right."

"Quite so, uncle. Burn carbon or charcoal either in the air or in oxygen, and it is sure always to make carbonic acid, and nothing else, if it is dry. No dew or mist gathers in a cold glass jar if you burn dry charcoal in it. The charcoal goes entirely into carbonie acid gas, and leaves nothing behind but ashes, which are only earthly stuff that was in the charcoal, but not part of the charcoal itself. And now, shall I tell you something about carbon ?"

you

"With all my heart," assented Mr. Bagges.

away,

"I said that there was carbon or charcoal in all common lights, so there is in every common kind of fuel. If heat coal or wood away from the air, some gas comes and leaves behind coke from coal, and charcoal from wood; both carbon, though not pure. Heat carbon as much as you will in a close vessel, and it does not change. in the least; but let the air get to it, and then it burns and flies off in carbonic acid gas. This makes carbon so convenient for fuel. But it is ornamental as well as useful, uncle. The diamond is nothing else than carbon."

"The diamond, eh? You mean the black diamond."

No; the diamond, really and truly. The diamond is only carbon in the shape of a crystal."

"Eh? and can't some of your clever chemists crystallise a little bit of carbon, and make a Koh-i-noor ?"

Ah, uncle, perhaps we shall some day. In the meantime I suppose we must be content with making carbon so brilliant as it is in the flame of a candle. Well; now you see that candle-flame is vapour burning, and the vapour,