4296. Because the candle will be extinguished, if the mine contains carbonic acid gas; but if the candle is not extinguished, the mine is safe, and the man may fearlessly descend.
4297. Why does a crowded room produce headache?
4298. Because we breathe the air vitiated by the crowd.
4299. Why is the air of a room vitiated by a crowd?
4300. Because it is deprived of its due proportion of oxygen, and laden with carbonic acid.