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._

Why do you have to strike a match to make it burn?

How does pulling the trigger make a gun go off?

What makes cooked foods taste different from raw ones?

Has it struck you as strange that we do not all burn up, since burning is a combining with oxygen, and we are walking around in oxygen all the time? The only reason we do not burn up is that it usually requires heat to start a chemical change. You already know this in a practical way. You know that you have to rub the head of a match and get it hot before it will begin to burn; that gunpowder does not go off unless you heat it by the sudden blow of the gun hammer which you release when you pull the trigger; that you have to concentrate the sun"s rays with a magnifying gla.s.s to make it set a piece of paper on fire; and that to change raw food into food that tastes pleasant you have to heat it. If heat did not start chemical change, you could never cook food,--partly because the fire would not burn, and partly because the food would not change its taste even if heated by electricity or concentrated sunlight.

Here is an experiment to show that gas will not burn unless it gets hot enough:

EXPERIMENT 97. Hold a wire screen 2 or 3 inches above the mouth of a Bunsen burner. Turn on the gas and light a match, holding the lighted match _above_ the screen. Why, do you suppose, does the gas below the screen not burn? Hold a lighted match to the gas below the screen. Does it burn now?

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:

[Ill.u.s.tration: 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?

[Ill.u.s.tration: 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.

You will be able better to understand why all this would happen after you do the following experiments, the first of which will show that light helps the chemical change called bleaching or fading.

EXPERIMENT 99. Rinse two small pieces of light-colored cloth.

(Lavender is a good color for this experiment.) Lay one piece in the bright sun to dry; dry the other in a dark cabinet or closet. The next day compare the two cloths. Which has kept its color the better? If the difference is not marked, repeat the experiment for 2 or 3 days in succession, putting the same cloth, wet, in the sun each time.

Bleaching is usually a very slow kind of burning. It is the dye that is oxidized (burned), not the cloth. Most dyes will combine with the oxygen in the air _if they are exposed to the sunlight_. The dampness quickens the action.

WHY SOME PEOPLE FRECKLE IN THE SUN. When the sunlight falls for a long time on the skin, it often causes the cells in the under part of the skin to produce some dark coloring matter, or pigment. This dark pigment shows through the outer layer of skin, and we call the little spots of it _freckles_. Some people are born with these pigment spots; but when the freckles come out from long exposure to the sunlight, they are an example right in our own skins of chemical change caused by the action of light. Tan also is due to pigment in the skin and is caused by light.

The next experiments with their explanations will show you how cameras can take pictures. If you are not interested in knowing how photographs are made, do the experiments and skip the explanations down to the middle of page 332.

EXPERIMENT 100. Dissolve a small crystal of silver nitrate (AgNO_3) in about half an inch of pure water in the bottom of a test tube. Distilled water is best for this purpose.

Now add one drop of hydrochloric acid (HCl). The white powder formed is a silver salt, called _silver chlorid_ (AgCl); the rest of the liquid is now a diluted nitric acid (HNO_3).

Pour the suspension of silver chlorid (AgCl) on a piece of blotting paper or on a paper towel, so that the water will be absorbed. Spread the remaining white paste of silver chlorid (AgCl) out over the blotter as well as you can. Cover part of it with a key (or anything that will shut off the light), and leave the other part exposed. If the sun is shining, put the blotter in the sunlight for 5 minutes. Otherwise, let as much daylight fall on it as possible for about 10 minutes. Now take the key off the part of the silver chlorid (AgCl) that it was covering and compare this with the part that was exposed to the light. What has the light done to the silver chlorid (AgCl) that it shone on?

What has happened is that the light has made the silver (Ag) _separate_ from the chlorine (Cl) of the silver chlorid (AgCl).

Chemists would write this:

AgCl -> Ag + Cl.

That is, silver chlorid (AgCl) has changed into silver (Ag) and chlorine (Cl). Chlorine, as you know, is a poisonous gas, and it floats off in the air, leaving the fine particles of silver behind.

When silver is divided into very tiny particles, it absorbs light instead of reflecting it; so it looks dark gray or black.

HOW PHOTOGRAPHS ARE MADE. All photography depends on this action of light. The plates or films are coated with a silver salt,--usually a more sensitive salt than silver chlorid. This is exposed to the light that shines through the lens of the camera. As you have learned, the lens brings the light from the object to a focus and makes an image on the film or plate. The light parts of this image will change the silver salt to silver; the dark parts will not change it. So wherever there is a white place on the object you are photographing, there will be a dark patch of silver on the film or plate, and wherever there is a dark spot on the object, there will be no change on the film or plate.

[Ill.u.s.tration: FIG. 173. The silver salt on the paper remains white where it was shaded by the key.]

As a matter of fact, the film or plate is exposed such a short time that there is not time for the change to be completed. So the photographer develops the negative; he washes it in some chemicals that finish the process which the light started.

If he exposed the whole plate to the light now, however, all the _unchanged_ parts of the silver salt would also be changed by the light, and there would be no picture left. So before he lets any light shine on it, except red light which has no effect on the silver salt, he dissolves off all the white unchanged part of the silver salt, in another kind of chemical called the _fixing bath_. This is called "fixing" the negative.

The only trouble with the picture now is that wherever there should be a patch of white, there is a patch of dark silver particles; and wherever there should be a dark place, there is just the clear gla.s.s or celluloid, with all the silver salt dissolved off. This kind of picture is called a _negative_; everything is just the opposite shade from what it should be. A white man dressed in a black suit looks like a negro dressed in a white suit.

HOW A PHOTOGRAPHIC PRINT IS MADE. The negative not only has the lights and shadows reversed, but it is on celluloid or gla.s.s, and except for moving pictures and stereopticons, we usually want the picture on paper. So a print is made of the negative. The next experiment will show you how this is done.