The reason the screen kept the gas below it from catching fire although the gas above it was burning was this: The heat from the flame above was conducted out to the sides by the wire screen as soon as it reached the screen; so very little heat could get through the screen to the gas below. Therefore the gas below the screen never got hot enough for the chemical change of oxidation, or burning, to take place. So the gas below it did not catch fire.
Another simple experiment with the Bunsen burner, that shows the same thing in a different way, is this:
Fig. 171. Why doesn't the flame above the wire gauze set fire to the gas below?
Experiment 98. Light the Bunsen burner. Open the air valve at the bottom all the way. Hold the wood end of a match (not the head) in the center of the inner greenish cone of flame, about half an inch above the mouth of the burner. Does the part of the match in the center of the flame catch fire? Does the part on the edge? What do you suppose is the reason for this? Where are the cold gas and air rushing in? Can they get hot all at once, or will they have to travel out or up a way before they have time to get hot enough to combine?
Fig. 172. The part of the match in the middle of the flame does not burn.
Application 73. Explain why boiled milk has a different taste from fresh milk; why blowing on a match will put it out; why food gets black if it is left on the stove too long.
Inference Exercise
Explain the following:
451. When you want bread dough to rise, you put it in a warm place.
452. Ink left long in an open inkwell becomes thick.
453. A ball bounces up when you throw it down.
454. When the warm ocean air blows over the cool land in the early morning, there is a heavy fog.
455. Striking a match makes it burn.
456. When you have something hard to cut, you put it in the part of the scissors nearest the handles.
457. A magnet held over iron filings makes them leap up.
458. Dishes in which flour thickening or dough has been mixed should be washed out with cold water.
459. A woolen sweater is liable to stretch out of shape after being washed.
460. When a telegraph operator presses a key in his set, a piece of iron is pulled down in the set of another operator.
Section 49. Chemical change caused by light.
How can a camera take a picture?
Why does cloth fade in the sun?
What makes freckles?
If light could not help chemical change, nothing would ever fade when hung in the sun; wall paper and curtains would be as bright colored after 20 years as on the day they were put up, if they were kept clean; you would never become freckled, tanned, or sunburned; all photographers and moving-picture operators would have to go out of business; but worst of all, every green plant would immediately stop growing and would soon die. Therefore, all cows and horses and other plant-eating animals would die; and then the flesh-eating animals would have nothing to eat and they would die; and then all people would die.