"Put a rubber finger cot on the finger and keep all air out," one friend advised me. "Air causes decay and will therefore be bad for the burn." "He's wrong; you should bandage it with clean cloth; you want air to reach the finger, I've heard," said another friend. "Oh, no, you don't; air makes things burn, and the burn will therefore get worse," still another one said. What should I have done? _APPLICATION 71._ Two students were discussing how coal was formed. "The trees must have fallen into water and been completely covered by it, or they would have decayed," said one. "Water makes things decay more quickly; there must have been a drought and the trees must have fallen on dry ground," said the second. Which was right? [Illustration: FIG. 169. Regulating the air opening in a gas stove.] _APPLICATION 72._ A gas stove had a yellow flame. In front, by the handles, was a metal disk with holes so arranged that turning it to the left allowed air to mix with the gas on the way to the flame, and turning it to the right shut the air off (see Fig. 170). One member of the family said, "Turn the disk to the left and let more air mix with the gas." But another objected. "It has too much air already; that's why the flame is yellow. Turn it to the right and shut off the air from below." "You're both wrong. Why do you want to change it?" said a third member of the family. "The yellow flame is the hottest, anyway. Can't you see that the yellow flame gives more light? And don't you know that light is just a kind of radiant heat? Of course the yellow flame is the hottest. Leave the stove alone." Who was right? [Illustration: FIG. 170. The air openings in the front of a gas stove.] INFERENCE EXERCISE Explain the following: 441. Iron tracks are welded together with an electric arc. 442. The cool mirror in a bathroom becomes covered with moisture when you take a hot bath. 443. This prevents you from seeing yourself in the mirror. 444. Carbon dioxid has oxygen in it, yet a burning match dropped into a bottle of it will go out. 445. A ship that sinks to the bottom of the ocean does not decay. 446. When women put their hair in curlers, they usually moisten the hair slightly. 447. To dry a pan after washing it, a person often sets it on the hot stove for a few minutes. 448. When you put a kettle of cold water over a gas flame, drops of water appear on the lower part of the sides of the kettle. 449. Electric power plants are often situated where running water will turn the dynamo. Explain the necessity of turning the dynamo. 450. We make carbon dioxid by burning carbon, but you cannot put different things together to make carbon. SECTION 48. _Chemical change caused by heat._