"We have been learning more about coal-gas to- day, Norah," said Willie. "Let us call Fred in and have a chat about it."Fred came in, and began by asking Norah to tell all she could remember about this gas.
"It is an invisible gas," she said. "It has a powerful and unpleasant smell, and it burns with a bright flame.""You should have been with us in school," said Fred. "Teacher showed us another property of this coal-gas. He had a soda-water bottle, filled with a mixture of air and coal-gas.""Oh yes," said Norah, "I remember. When thecork is pulled out and the light is brought near, the gas explodes with a great bang. You told me about it when we were talking about coal.""So I did," said Fred. "I am glad you remember all about it. Teacher told us that sometimes people are careless, and leave one of the gas-burners turned on, so that the gas escapes into the room. Then wehave all the materials ready for an explosion on a terrible scale. All that is wanted is a light.
"You must never take a light into a room if you can smell gas. You are quite safe without a light, no matter how much gas has escaped.
"Go in, open the doors and windows, and the gas will soon pass away out of the room into the open air, and fresh air will take its place. Then, and not till then, will it be safe to get a light, and find out where the gas is escaping.""You remember the fire-damp in the mine," said Willie.
"Oh yes," said Norah. "The coal gives of this gas in great quantities.""Teacher says that this gas is really the same as the coal-gas we are speaking about,and when it mixes with the
air in the mine it becomes terribly explosive.""I daresay you remember the Davy Lamp which theminers use," said Fred. "Teacher has got one, and he showed us what it is that makes it safe for burning in the mine.
"He held a piece of wire gauze two or three inches above the gas-burner, and turned on the tap.
Of course the gas passed up through the gauze. When he put a light near, the gas took fire above the gauze, but it did not burn at all below it.
"In the Davy Lamp, you know, the flame is shut in with this wire gauze. The explosive gas, if there is any in the mine, passes through into the lamp and burns round the flame, but the flame itself cannot pass out through the gauze, and so the air in the mine is safe from explosion."SUMMARY
Coal-gas is invisible. It has a powerful, unpleasant smell, and it burns with a bright ?ame. It is very explosive if mixed with air. This is the ?re-damp of the coal-mine. The Davy lamp is safe, because, although the explosive gas can enter through the gauze, and burn round the flame, the flame itself cannot pass out through the gauze.
Lesson 33
