"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.
