"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.
