"Every lesson we have," said Willie, "seems to show us, more and more, what a wonderful world we live in. How lucky we are to be learning all these things. Whoever would have thought, now, that poisonous carbonic acid gas could be of any use? Why, it would kill us and every living animal if we were to breathe it for only two or three minutes. You remember our chats about the coal mine, Norah, and the terrible explosions of fire-damp that sometimes happen?""Oh yes. I shall never forget those dreadful places," said Norah.
"Well, teacher told us that after the explosion takes place," said Willie, "dense masses of carbonic acid gas are formed. The miners call the gas after- damp or choke-damp. Dreadful as the first danger is, more are killed afterwards-suffocated by the choke-damp-than by the explosion.""But do you mean to say that this poisonous gascan be of any use?" asked Norah.
"Let me tell you," said Fred. "You know that the breathing of animals and the burning of fires are constantly sending out some of this gas into the air. There is always carbonic gas in the air.""Of course there must be," said Norah. "It seems to me that with fires and animals always making it, there must be a very large quantity of this gas in the air.""Ah!" said Fred, "that is just the most wonderful part of it. Teacher showed us a saucer which he had filled with clear lime-water during the morning. By the time we saw it there was the same milky, cloudy-looking film on its surface, as we saw in the lime-water when we breathed into it the other day.
"That proved of course that carbonic acid gashad been acting on the lime-water. But the water below was quite clear, and there was only a film on the surface. That tells us clearly that after all there is only a very small quantity of carbonic acid gas in the air.
"Now, as we know that such a large amount of this gas is being constantly sent into the air, we must begin to wonder what becomes of it.
"Teacher told us all about it. Plants of every kind take in from the air carbonic acid gas, and use it to build up their own substance. It is their food.
"This is why there is never very much carbonic acid gas in the air. As quickly as it is formed by breathing and burning, it is taken in by plants as their food."SUMMARY
The breathing of animals and burning of all sorts are constantly loading the air with carbonic acid gas. Plants breathe in this carbonic acid through the breathing pores of their leaves. It is part of their food.
Lesson 45
