"We have already learned that the air is a mixture of the two gases, oxygen and nitrogen," said Mr. Wilson. "We could make some air for ourselves by preparing one bottle of oxygen and four similar bottles of nitrogen, and mixing them together. The mixture would be pure air.
"But the air all round us is never a perfectly pure mixture of these two gases. I think you know something else which it contains.""Air always contains carbonic acid gas, sir," said Fred. "Every fire, every furnace, and every gas-jet that burns, helps to load the air with carbonic acid, and so do all the animals that live and breathe on the earth.""Then, too, there is always water-vapor in the air, sir," said Willie.
"You are, both of you, quite
right," said Mr. Wilson. "Let us deal first with the carbonic acid gas. The other day we burned a piece of carbon in a jar of oxygen. We will now burn this taper in oxygen. The taper burnsfiercely for a time and then goes out. It goes out because all the oxygen has been consumed in the burning. There is no oxygen in the jar now; in place of it there is carbonic acid gas.
"How shall I prove that there is carbonic acid gas in the jar?""Pour some lime-water into the bottle, sir," said Fred. "Right. I will do it now, and you see the lime-water atonce becomes cloudy or milky-looking. If we had tested the jar in the same way, after burning the carbon in it, the result would have been the same, for whenever carbon burns, carbonic acid gas is produced.
"Then it is clear that the taper must contain carbon. Indeed all things that burn-wood, coal, coke, fuel of every kind, as well as candles, oil, coal-gas-contain carbon. It is the carbon in them which makes them valuable as substances for burning, and the burning always produces carbonic acid gas.
"Now how shall we prove that carbonic acid gas is given off with our breath?""If you breathe into lime-water," said Fred, "the lime-water willprove the presence of carbonic acid gas by turning cloudy.""Quite right, Fred," said
Mr. Wilson. "We are now quite clear as to the sourcesof carbonic acid gas in the air. Yet with all the fires and lights, and all the many millions of breathing animals and people, there is never more than a mere trace of carbonic acid gas in ordinary air-usually about four parts in 1000. How is this?""Every plant that grows, sir," said Fred, "requires carbonic acid gas. The plants take carbonic acid from the air, and so a balance is kept between the wants of animals and plants all over the world."Lesson 39