Fig. 173.The silver salt on the paper remains white where it was shaded by the key.

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 silveron the film or plate, and wherever there is a dark spot on the object, there will be no change on the film or plate.

Fig. 173.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 theunchangedparts 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 dissolvesoff all the white unchanged part of the silver salt, in another kind of chemical called thefixing 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 glass or celluloid, with all the silver salt dissolved off. This kind of picture is called anegative; 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 glass, 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.

Experiment 101.In a dark room or closet, take a sheet of blueprint paper from the package, afterwards closing the package carefully so that no light can get to the papers inside. Hold the piece of blueprint paper under your waist or coat, to keep it dark when you go into the light. Now lay it, greenish side downward, on a negative. Hold the two together, or place them in a printing frame, and turn them over so that the light will shine through the negative upon the greenish side of the blueprint paper. Be sure that the paper is held firmly against the negative and not moved around. Let the sun shine through the negative upon the paper for 1 or 2 minutes according to the brightness of the sun, or let the gray light of the sky, if it is cloudy, shine on it for 5 or 10 minutes. Now quickly put the blueprint paper (not the negative) into a basin of water, facedown. Wash for a couple of minutes. Turn it over and examine it. If it has been exposed to the light too long, it will be dark; if it has been exposed too short a time, it will be too light; in either case, if the print is not clear, repeat with a fresh piece of blueprint paper, altering the time of exposure to the sunlight to improve the print.Figs.Figs.174 and 175. Where the negative is dark, the print is light.You can make pretty outline pictures of leaves and pressed flowers, or of lace, by laying these on the blueprint paper in place of the negative and in other respects doing as directed above.

Figs.Figs.174 and 175. Where the negative is dark, the print is light.

You can make pretty outline pictures of leaves and pressed flowers, or of lace, by laying these on the blueprint paper in place of the negative and in other respects doing as directed above.

In making blueprints you are changing an iron salt instead of a silver salt, by the action of light. Regular photographic prints are usually made on paper treated with a silver salt rather than with iron salt, and sometimes a gold or platinum salt is used. But these other salts have to be washed off with chemicals since they do not come off in water, as the unchanged part of the iron salt comes off when you fix the blueprint paper in the water bath.

Since the light cannot get through the black part of a negative, the coating on the paper behind that part is not affected and it stays light colored; and since the light can get through the clear parts of the negative, the coating on the paper back of those partsisaffected and becomes dark. Therefore, the print is "right side out,"—there is a light place on the print for every white place on the object photographed, and there is a dark place on the print for every black place on the object.

Moving-picture films are printed from one film to another, just as you printed from a negative to a piece of paper. The negative is taken on one film, then this is printed on another film. The second film is "right side out."

Light and the manufacture of food in plants.Much the most important chemical effect of light, however, is not in making photographs, in bleaching things, or in "burning" your skin. It is in the putting together of carbon and water to make sugar in plants. Plants get water (H2O) from the earth and carbon dioxid (CO2) from the air. When the sun shines on chlorophyll, the green substance in plants, the chlorophyll puts them together and makes sugar. The plant changes this sugar into starch and other foods, and into the tissues of the plant itself. Nothing in the world can put carbon dioxid and water together and make food out of them except certain bacteria and the chlorophyll of plants. And light is absolutely necessary for this chemical action. Try this experiment:

Experiment 102.Pin together two pieces of cork on opposite sides of a leaf that is exposed to the sun. The nextday take this leaf from the plant and heat it in a beaker of alcohol until the green coloring matter is removed from the leaf. Then place the leaf in a glass of water that contains iodine. The iodine will color the leaf dark where the cells contain starch. (See Experiment 115, page373.) Is starch formed where the light does not reach the leaf?

No plant can make food except with the help of light. The part of the plant that can put carbon dioxid and water together is the green stuff or chlorophyll, and this can work only when light is shining on it. So all plants would die without light.

But if all plants should die, all animals would die also, for animals cannot make food out of carbon dioxid and water, as they do not have the chlorophyll that puts these things together. A lion does not live on leaves, it is true, but he lives on deer and other animals that do live on leaves and plants. If the plants died, all plant-eating animals would die. Then there would be nothing for the flesh-eating animals to eat except each other, and in time no animals would be left in the world. The same thing would happen to the fish. And man, of course, could no longer exist. The food supply of the world depends on the fact that light can start chemical change.

Oxygen released in the manufacture of plant food.Besides in one way or another giving us all of our food, plants, helped by light, also give us most of the free oxygen that we breathe. We and all animals get the energy by which we live bycombiningoxygen with the hydrogen of our food (forming water) and by combining oxygen with the carbon in our food (formingcarbon dioxid). This combining (burning or oxidizing) gives us our body heat and the energy to move. The free oxygen is carried to the different parts of our bodies by the red blood corpuscles that float in the liquid part of the blood. The liquid part of the blood also carries the food to the different parts of the body, and the food contains the carbon and hydrogen that is to be burned. Then in a muscle, for instance, the oxygen that has been carried by the corpuscles combines with the carbon to form carbon dioxid, and with the hydrogen to form water. The corpuscles carry part of the carbon dioxid back to the lungs, and the water is carried with other wastes and the rest of the carbon dioxid in the liquid part of the blood. In the lungs the carbon dioxid is exchanged for the free oxygen we have just inhaled, and we exhale the carbon dioxid. A good deal of water is also breathed out, as you can tell from the way the mist gathers on a window pane when you blow on it.

If there were only animals (including people) in the world, all the free oxygen in the air would in time be combined by the animals with hydrogen to make water and with carbon to make carbon dioxid (CO2). As animals cannot breathe water and cannot get any good from carbon dioxid, they would all smother.

But the plants, as we have already said, use carbon dioxid (CO2) and water (H2O) to make food. They do not need so much oxygen, and so they set some of it free. The countless plants in the world set the oxygen free as rapidly as the countless animals combine it with hydrogen to make water and with carbon to make carbon dioxid. Since the water and carbondioxid are the main things a plant needs to make its food, the animals really are as helpful to the plants as the plants are to the animals. For the animals furnish the materials to the plants for making their food in exchange for the ready-made food furnished by the plant. And both plants and animals would die if light stopped helping to bring about chemical change.

Application 74.Explain why the heart of a cabbage is white instead of green like the outside leaves; why a photographer works in a dark room with only a ruby light; why you get freckled in the sun.

Explain the following:461. If a pin is put through a lamp cord, a fuse is likely to blow out.462. The wall paper back of a picture is often darker than that on the rest of the wall.463. If you wet an eraser, it rubs through the paper.464. Clothes are hot after being ironed.465. If you drop candle grease on your clothes, you can remove the grease by placing a blotter over it and pressing the blotter with a warm iron.466. Milliners cover hats that are on display in windows where the sun shines in on the hats.467. You pull down on a rope when you try to climb it.468. In taking a picture, you expose the sensitive film or plate to the light for a short time.469. Good cameras have an adjustable front part so that the lens may be moved nearer to the plate or film, or farther from it, according to the distance of the object to be photographed.470. A pencil has to be resharpened frequently when it is much used.

Explain the following:

461. If a pin is put through a lamp cord, a fuse is likely to blow out.

462. The wall paper back of a picture is often darker than that on the rest of the wall.

463. If you wet an eraser, it rubs through the paper.

464. Clothes are hot after being ironed.

465. If you drop candle grease on your clothes, you can remove the grease by placing a blotter over it and pressing the blotter with a warm iron.

466. Milliners cover hats that are on display in windows where the sun shines in on the hats.

467. You pull down on a rope when you try to climb it.

468. In taking a picture, you expose the sensitive film or plate to the light for a short time.

469. Good cameras have an adjustable front part so that the lens may be moved nearer to the plate or film, or farther from it, according to the distance of the object to be photographed.

470. A pencil has to be resharpened frequently when it is much used.

Section 50.Chemical change caused by electricity.

How are storage batteries charged?How is silver plating done by electricity?

How are storage batteries charged?

How is silver plating done by electricity?

You have already done an experiment showing that electricity can start chemical change, for you changed water into hydrogen and oxygen by passing a current of electricity through the water.

The plating of metals is made possible by the fact that electricity helps chemical change. You can nickel plate a piece of copper in the following manner:

Experiment 103.Dissolve a few green crystals of "double nickel salts" in water, until the water is a clear green. The water should be about 2 or 3 inches deep in a glass or china bowl that is not less than 5 inches across.Lay two bare copper wires across the bowl, about 3 inches apart, as shown in Figure 177. Connect the positive wire from a storage battery, or the wire from the carbon of a battery of three or four cells, to an end of one bare wire. Connect the negative wire from the storage or the negative wire from the zinc of the other battery to an end of the second bare wire.Fig. 176.Fig. 176.The copper and the nickel cube ready to hang in the cleansing solution.Now fasten a fine bare wire 5 or 6 inches long around a small piece of copper, and another like it around a piece ofnickel, as shown in Figure 176. Then put the piece of copper in the bottom of an evaporating dish, with the wire hanging out, as in Figure 177.

Lay two bare copper wires across the bowl, about 3 inches apart, as shown in Figure 177. Connect the positive wire from a storage battery, or the wire from the carbon of a battery of three or four cells, to an end of one bare wire. Connect the negative wire from the storage or the negative wire from the zinc of the other battery to an end of the second bare wire.

Fig. 176.Fig. 176.The copper and the nickel cube ready to hang in the cleansing solution.

Now fasten a fine bare wire 5 or 6 inches long around a small piece of copper, and another like it around a piece ofnickel, as shown in Figure 176. Then put the piece of copper in the bottom of an evaporating dish, with the wire hanging out, as in Figure 177.

Fig. 177.Fig. 177.Cleaning the copper in acids.

Pour over the piece of copper enough of the cleansing solution to cover it.9The cleansing solution contains strong acids. If you get any on your skin or clothes, wash it off immediately with ammonia or soda.As soon as the copper is bright and clean, take it out of the cleansing solution and suspend it by thenegativewire in the green nickel solution. You can tell if you have it on the negative wire, for in that case bubbles will rise from itduring the experiment. The copper should be entirely covered by the nickel solution, but should not touch the bottom or sides of the bowl. Pour the cleansing solution from the evaporating dish back into the bottle. Suspend the nickel, in the same way as the copper, from thepositivewire crossing the bowl. When set up, the apparatus should appear as shown in Figure 178.

Footnote 9:The formula for making the cleansing solution is as follows:1 cup water.1 cup concentrated sulfuric acid.1 cup concentrated nitric acid.1 teaspoonful concentrated hydrochloric acid.The sulfuric and nitric acids must be measured in glass or china cups, and the hydrochloric acid must be measured in a silver-plated spoon or in glass—not in tin.

The formula for making the cleansing solution is as follows:

1 cup water.

1 cup concentrated sulfuric acid.

1 cup concentrated nitric acid.

1 teaspoonful concentrated hydrochloric acid.

The sulfuric and nitric acids must be measured in glass or china cups, and the hydrochloric acid must be measured in a silver-plated spoon or in glass—not in tin.

Fig. 178.Fig. 178.Plating the copper by electricity.

Turn on the electricity. If the copper becomes black instead of silvery, clean it again in the cleansing solution, and move the two bare wires much farther apart,—practically the full width of the bowl. If the copperstill turns black, it means that too much electricity is flowing. In that case use fewer batteries.

The electricity has started two chemical changes. It has made part of the piece of nickel combine with part of the solution of nickel salt to form more nickel salt, and it has made some of the nickel salt around the copper change into metallic nickel. Then the negative electricity in the copper has attracted the positive bits of nickel metal made from the nickel salt, and made them cling to the copper. If there is no dirt or grease on the copper, the particles of nickel get so close to it that they stick by adhesion, even after the electric attraction has ceased. This leaves the copper nickel-plated, but to make it shiny the nickel plating must be polished.

Silver plating and gold plating are done substantially in the way that you have done the nickel plating, only gold salt or silver salt is used instead of nickel salt.

Just as electricity helps chemical changes in plating, it helps changes in a storage battery. But in the storage battery the new compounds formed by "charging" the battery change back again and generate electricity when the poles of the battery are connected with each other by a good conductor.

Application 75.Explain how spoons can be silver plated; how water can be changed into hydrogen and oxygen.

Explain the following:471. Clothes dry best in the sun and wind.472. Proofs of photographs that have not been thoroughly "fixed" fade if left out of their envelope.473. Blowing a match puts it out, yet a good draft is necessary for a hot fire.474. A cup does not naturally fall apart, yet after it is broken it falls apart even if you fit the pieces together again.475. Crayon leaves marks on a blackboard.476. A baked potato tastes very different from a raw one.477. An air-filled automobile tire is harder at noon than in the early morning.478. When a live trolley wire breaks and falls to the street, it becomes so hot that it burns.479. Glass jars of fruit should be kept in a fairly dark place.480. You wash dishes inhotwater.

Explain the following:

471. Clothes dry best in the sun and wind.

472. Proofs of photographs that have not been thoroughly "fixed" fade if left out of their envelope.

473. Blowing a match puts it out, yet a good draft is necessary for a hot fire.

474. A cup does not naturally fall apart, yet after it is broken it falls apart even if you fit the pieces together again.

475. Crayon leaves marks on a blackboard.

476. A baked potato tastes very different from a raw one.

477. An air-filled automobile tire is harder at noon than in the early morning.

478. When a live trolley wire breaks and falls to the street, it becomes so hot that it burns.

479. Glass jars of fruit should be kept in a fairly dark place.

480. You wash dishes inhotwater.

Section 51.Chemical change releases energy.

Why is fire hot?What makes glowworms glow?Why does cold quicklime boil when you pour cold water on it?

Why is fire hot?

What makes glowworms glow?

Why does cold quicklime boil when you pour cold water on it?

If no energy were released by chemical change, we should run down like clocks, and could never be wound up again. We could breathe, but to do so would do us no more good than it would if oxygen could not combine with things. Oxidation would go on in our bodies, but it would neither keep us warm nor help us to move. A few spasmodic jerks of our hearts, a few gasps with our lungs, and they would stop, as the muscles would have no energy to keep them going.

The sunlightmightcontinue to warm the earth, as we are not sure that the sun gets any of its heat from chemical change. But fires, while they would burn for an instant, would be absolutely cold; no energy would be given out by the fuel combining with oxygen. But the fires could not burn long, because there would be nothing to keep the gases and fuel hot enough to make them combine with the oxygen.

Even during the instant that a fire lasted it would beinvisible, for it would give off no light if no energy were released by the chemical change. Only electric lights and heaters would continue to work, and even some of these would fail. The electric motors in submarines and electric automobiles would instantly stop; battery flashlights would go out as quickly as the fire; no doorbells would ring. In short, all forms of electric batteries would stop sending currents of electricity out through their wires, and everything depending upon batteries would stop running.

A fire gives out heat and light; both are kinds of energy. And it is the electric energy caused by the chemical change in batteries that runs submarines, electric automobiles, flashlights, and doorbells. Since burning (oxidation) is simply a form of chemical change, it is not difficult to realize that chemical change releases energy.

Why glowworms glow.When a glowworm glows at night, or when the head of a match glows as you rub it on your wet hand in the dark, we call the lightphosphorescence. The name "phosphorus" means light-bearing, and anything like the element phosphorus, that glows without actively burning, is said to be phosphorescent. Match heads have phosphorus in them. Phosphorescence is almost always caused by chemical change. The energy released is a dim light, not heat or electricity. Sometimes millions of microscopic sea animals make the sea water in warm regions phosphorescent. They, like fireflies, glowworms, and will-o'-the-wisps, have in them some substance that is slowly changing chemically, and energy is released in the form of dim light as the change takes place. Most luminouspaint is phosphorescent for the same reason,—there is a chemical change going on that releases energy in the form of light.

When you poured the hydrochloric acid on the zinc to make hydrogen, the flask became warm; the chemical change going on in the flask released heat energy.

Application 76.Explain why pouring cold water on cold quicklime makes the slaked lime that results boiling hot; why a cat's eyes shine in the dark; why a piece of carbon and a piece of zinc placed in a solution of sal ammoniac will make electricity run through the wire that connects them; why fire is hot.

Explain the following:481. A baking potato sometimes bursts in the oven.482. Turpentine is used in mixing paint.483. Sodium is a metal; chlorine is a poisonous gas; yet salt, which is made up of these two, is a harmless food.484. When bricklayers mix water with cement and lime, the resulting mortar boils and steams.485. Green plants will not grow in the dark.486. Parts of the body are constantly uniting with oxygen. This keeps the body warm.487. Water will not always put out a kerosene fire.488. Delicately colored fabrics should be hung in the shade to dry.489. A match glows when you rub it in the dark.490. Candy hardens when it cools.

Explain the following:

481. A baking potato sometimes bursts in the oven.

482. Turpentine is used in mixing paint.

483. Sodium is a metal; chlorine is a poisonous gas; yet salt, which is made up of these two, is a harmless food.

484. When bricklayers mix water with cement and lime, the resulting mortar boils and steams.

485. Green plants will not grow in the dark.

486. Parts of the body are constantly uniting with oxygen. This keeps the body warm.

487. Water will not always put out a kerosene fire.

488. Delicately colored fabrics should be hung in the shade to dry.

489. A match glows when you rub it in the dark.

490. Candy hardens when it cools.

Section 52.Explosions.

What makes a gun shoot?What makes an automobile go?

What makes a gun shoot?

What makes an automobile go?

Usually we think of explosions as harmful, and they often are, of course. Yet without them we could no longer run automobiles; gasoline launches would stop at once; motorcycles would no longer run; gasolineengines for pumping water or running machinery would not be of any use; and all aviation would immediately cease. Tunneling through mountains, building roads in rocky places, taking up tree stumps, and preparing hard ground for crops would all be made very much more difficult. War would have to be carried on much as it was during the Middle Ages; soldiers would use spears and bows and arrows; battleships would be almost useless in attacking; modern forts would be of little value; cannon, guns, rifles, howitzers, mortars, and revolvers would all be so much junk.

Fig. 179.Fig. 179.The explosion of 75 pounds of dynamite. A "still" from a motion-picture film.

Fig. 180.Fig. 180.Diagram of the cylinder of an engine. The piston is driven forward by the explosion of the gasoline in the cylinder.

What makes an automobile go.In all the above cases the explosions are caused by chemical action. When gasoline mixed with air is sprayed into the cylinder of an automobile, an electric spark makes the gasoline combine with the oxygen of the air; the gasoline suddenly burns and changes to steam and carbon dioxid. As you already know, when a liquid like gasoline turns to gases such as steam and carbon dioxid, the gases take much more room. But that is not all that happens. Much heat is released by the burning of the gasoline spray, and heat causes expansion. So the gases formed by the burning gasoline are still further expanded by the heat released by the burning. Therefore they need a great deal more room; but they are shut up in a small place in the top of a cylinder. The only thing to hold them up in this small space, however, is a piston (Fig. 180), and the suddenly expanding gases shove this piston down and escape. The piston is attached to thedrive wheel of the automobile, and when the piston is pushed down it gives the automobile a push forward. If it were not for the expansion of a gas in the cylinder, this gas being confined to a small space, the piston would not be pushed down.

An explosion is simply the sudden pushing of a confined gas expanding on its way to freedom. The gasoline vapor and air were the confined gas. Their chemical combining made them expand; by pushing the piston out of its way the newly formed gas suddenly freed itself. This was an explosion, and it gave the automobile one forward push. But the automobile engine is so arranged that the piston goes up into the cylinder again, and is pulled down again, drawing a spray of gasoline and air into the cylinder after it. Then it goes up a second time, an electric spark explodes the gasoline, the piston is forced down violently once more, and so it goes on. There are several cylinders, of course, and the explosions take place within them one after the other so as to keep the automobile going steadily.

How a gun shoots.Pulling a trigger makes a gun shoot by causing an explosion. There is a spring on the hammer of a gun. This drives the hammer down suddenly when you release the spring by pulling the trigger. The hammer jars the chemicals in the cap and causes them to explode. The heat and flame then cause the oxygen in the gunpowder to combine with some of the other elements in the powder to make a gas. The gas requires more room than the powder and is further expanded by the heat released by the chemical change. The expanding gas frees itself by pushing the bulletout of its way. The bullet gets such a push through the exploding of the gunpowder that it may fly to a mark and pierce it.

Fig. 181.Fig. 181.The most powerful explosions on earth occur in connection with volcanic activity. The photograph shows Mt. Lassen, California, the only active volcano in the United States.

There is a slight explosion even when you shoot an air gun. First you compress some air in the upper part of the barrel of the air gun; then you suddenly release it. The only thing in the way of the expanding air is the bullet; so the air pushes this out in front of it.

In Experiment 36, where you stoppered a test tube containing a little water and then held the tube over a flame until the cork flew out, you were causing an explosion. As the water changed to steam, the steam was an expanding gas. It was at first confined to the test tube by the cork. Then there was an explosion; the gas freed itself by blowing out the cork.

Steam boilers have safety valves to prevent explosions. These are valves so arranged that when the steam expands and presses hard enough to endanger the boiler, the steam will open the valves and escape instead of bursting the boiler to get free.

Explosives.Dynamite, gunpowder, and most explosives are mixtures of solids or liquids that will combine easily and will form gases that expand greatly as a result of the combination. One of the essentials in explosives is some compound of oxygen (such as the manganese dioxid or potassium chlorate you used to make oxygen in Experiment 93) which will easily set its oxygen free. This oxygen combines very swiftly with something else in the explosive, releasing heat and forming a gas that takes much more room. In its effort to free itself, this expanding gas will blast rocks out of the way, shoot cannon balls, or do any similar work.

But if gunpowder does not have to push anything of much importance out of its way to expand, there is no explosion. That is why a firecracker merely fizzes when you break it in two and light the powder. The cardboard no longer confines the expanding gas; so there is nothing to burst or to push violently out of the way.

Useful explosions are generally caused by a chemicalaction which suddenly releases a great deal of heat and combines solid things into expanding gases. But the bursting of a steam boiler, or the "blow out" of an automobile tire, or the bursting of a potato in the oven, although not caused by chemical action, still are real explosions. An explosion is thesuddenrelease of a confined gas.

Application 77.Explain how gasoline makes a motorcycle go, and why it goes "pop, pop, pop." Explain why a paper bag will burst with a bang, when you blow it up and then clap it between your hands; why a Fourth-of-July torpedo "goes off" when you throw it on the pavement.

Explain the following:491. The engine of an automobile is cooled by the water that passes over it from the radiator.492. When you light a firecracker, the flame travels down the wick until it reaches the gunpowder, and then the firecracker bursts with a bang.493. If you light a small pile of gunpowder in the open, as you do when you make a squib by breaking the firecracker in two, you merely have a blaze.494. Air-filled tires make bicycles ride much more evenly than solid tires would.495. When clay has once been baked into brick, you can never change it back to clay.496. A photographic negative turns black all over if it is exposed to the light before it is "fixed."497. The outside of a window shade fades.498. A vacuum electric lamp globe feels hot instantly when turned on, but if turned off again at once, it immediately feels cold.499. Coal gas is made by heating coal very hot in an air-tight chamber.500. White straw turns yellow when it is long exposed to the sunlight.

Explain the following:

491. The engine of an automobile is cooled by the water that passes over it from the radiator.

492. When you light a firecracker, the flame travels down the wick until it reaches the gunpowder, and then the firecracker bursts with a bang.

493. If you light a small pile of gunpowder in the open, as you do when you make a squib by breaking the firecracker in two, you merely have a blaze.

494. Air-filled tires make bicycles ride much more evenly than solid tires would.

495. When clay has once been baked into brick, you can never change it back to clay.

496. A photographic negative turns black all over if it is exposed to the light before it is "fixed."

497. The outside of a window shade fades.

498. A vacuum electric lamp globe feels hot instantly when turned on, but if turned off again at once, it immediately feels cold.

499. Coal gas is made by heating coal very hot in an air-tight chamber.

500. White straw turns yellow when it is long exposed to the sunlight.

Section 53.Chemical change helped by solution.

Why does iron have to get wet to rust?Is it good to drink water with your meals?

Why does iron have to get wet to rust?

Is it good to drink water with your meals?

When iron rusts, it is really slowly burning (combining with oxygen). If your house is on fire, you throw water on it to stop the burning. Yet if you throw water on iron it rusts, or burns, better than if you leave it dry. What do you suppose is the reason for this?

The answer is not difficult. You know perfectly well that iron does not burn easily; we could not make fire grates and stoves out of iron if it did. But when iron is wet, a little of it dissolves in the water that wets it. There is also a little oxygen dissolved in the water, as we know from the fact that fish can breathe under the water. Thisdissolvedoxygen can easily combine with thedissolvediron; thesolutionhelps the chemical change to take place. The chemical change that results is oxidation,—the iron combining with oxygen,—which is a slow kind of burning; and in iron this is usually calledrusting.10But when we pour water on burning wood, the woodstopsburning, for there is not nearly enough oxygen dissolved in water to combine rapidly with burning wood; and the water shuts off the outside air from burning wood and therefore puts the fire out.

Footnote 10: The rusting of iron is not quite as simple as this, as it probably undergoes two or three changes before finally combining with oxygen. But the solution helps all these changes.

Another chemical change, greatly helped by solution, is the combining of the two things that baking powderis made of, and the setting free of the carbon dioxid (CO2) that is in one of them. Try this experiment:

Experiment 104.Put half a teaspoonful of baking powder in the bottom of a cup and add a little water. What happens?

The chemical action which takes place in the baking powder and releases the gas in bubbles—the gas is carbon dioxid (CO2)—will not take place while the baking powder is dry; but when it is dissolved, the chemical change takes place in the solution.

If you ate your food entirely dry, you would have a hard time digesting it; and this would be for the same reason that baking powder will not work without water. Perhaps you can drink too much water with a meal and dilute the digestive juices too much; certainly you should not use water to wash down your food and take the place of the saliva, for the saliva is important in the digestion of starch. But you need also partly to dissolve the food to have it digest well. Crackers and milk are usually more easily digested than are plain crackers, for the milk partly dissolves the crackers, and drinking one or two glasses of water with a meal hastens the digestion of the food.

Application 78.Explain why paint preserves wood; why iron will rust more quickly in a wet place than it will either under water or in a dry place; why silver salts must be dissolved in order to plate a spoon by electricity.

Explain the following:501. There is dew on the grass early in the morning.502. Cold cream makes your hands and face soft.503. Glowworms and fireflies can be seen on the darkest nights.504. A lake looks gray on a cloudy day and blue on a clear day.505. Dried fruit will keep much longer than fresh fruit.506. If you scratch a varnished surface, you can rub the scratch out completely by using a cloth wet with alcohol.507. Soda is usually dissolved in a little water before it is added to a sour-milk batter.508. Iron rusts when it gets wet.509. Peroxide is usually kept in brown bottles.510. Dry lye may be kept in tin cans, but if the lye ismoistenedit will eat the can.

Explain the following:

501. There is dew on the grass early in the morning.

502. Cold cream makes your hands and face soft.

503. Glowworms and fireflies can be seen on the darkest nights.

504. A lake looks gray on a cloudy day and blue on a clear day.

505. Dried fruit will keep much longer than fresh fruit.

506. If you scratch a varnished surface, you can rub the scratch out completely by using a cloth wet with alcohol.

507. Soda is usually dissolved in a little water before it is added to a sour-milk batter.

508. Iron rusts when it gets wet.

509. Peroxide is usually kept in brown bottles.

510. Dry lye may be kept in tin cans, but if the lye ismoistenedit will eat the can.

Section 54.Acids.

Why are lemons sour?How do acids act?

Why are lemons sour?

How do acids act?

Some acids are very powerful. There is one, calledhydrofluoric acid, that will eat through glass and has to be kept in wax bottles; and all acids tend to eat or corrode metals. You saw what hydrochloric acid did to the zinc shavings when you wanted to make a balloon; or, to be more accurate, you saw what the zinc shavings did to the acid, for the hydrogen gas that bubbled off was driven out of the acid by the zinc. Then the zinc combined with the rest of the acid to form what chemists call asalt.

If we were to let the soft metal, sodium, act on hydrochloric acid, we should get hydrogen also; but the salt that formed would be regular table salt (NaCl). You cannot do this experiment, however, as the sodium does its work so violently that it is dangerous.

Experiment 105.To be done by the teacher before the class. If acid spatters on any one's skin or clothes, wash it of immediately with ammonia or a strong soda solution.

Fig. 182.Fig. 182.Etching copper with acid.

Drop a little candle grease on a piece of copper about 3/4 inch wide and 2 or 3 inches long. In the flame of a Bunsen burner, gently heat the end of the copper that has the candle grease (paraffin) on it, so that the paraffin will spread out all over the end. Let it harden. With a nail, draw a design in the paraffin on the copper, scratching through the thin coat of paraffin to the copper below. Pour a couple of drops of concentrated nitric acid on the paraffin-covered end of the piece of copper, and spread the acid with a match so that it can get down into the scratches. Let it stand by an open window for 5 or 10 minutes. Do not inhale the brown fumes that are given off. They are harmless in small amounts, but if breathed directly they are very irritating. Now wash off the acid by holding the copper under the hydrant, and scrape off the paraffin.

The nitric acid did to the copper in this experiment exactly what the hydrochloric acid did to the zinc shavingswhen you made the toy balloon. The copper drove the hydrogen out of the nitric acid and incidentally broke down some of the nitric acid to make the brown gas, and then the copper joined the rest of the nitric acid to make a salt calledcopper nitrate. This salt is green, and it dissolves in water. When you washed the copper, the green salt was washed away and a dent remained in the copper where the copper salt had been.

Here is a more practical experiment showing the action of acid on metal:

Experiment 106.Use two knives, one of bright steel and the other a silver-plated one. If the steel knife is not bright, scour it until it is. Drop a little lemon juice on each knife and let it stand for a few minutes, while the teacher does the next experiment. Then rinse both knives and examine them. What has the lemon juice done to the silver knife? to the steel one?

The lemon juice acts in this way because it is acid. Acids act on the taste nerves in the tongue and give the taste of sourness; everything sour is an acid. The black stuff formed on the steel knife by the lemon juice is an iron salt. The iron in the knife drove the hydrogen out of the lemon juice, but there was not enough for you to see it coming off; then the iron combined with the rest of the lemon juice to form an iron salt.

Whenever an acid acts on a metal, the metal drives off the hydrogen and forms a salt. The salt is not always good to eat; for instance, the salt that tin forms with acids is poisonous.

Action of acids on other substances.Acids do not act on metals only, however. Watch the next experiment to see what a strong acid will do to cloth.

Experiment 107.To be done by the teacher.Put a drop of concentrated nitric or sulfuric acid on a piece of colored wool cloth, or on a piece of colored silk. Let it stand for a few minutes, then rinse it thoroughly. Test the cloth where the acid has been to see whether or not it is as strong as the rest of the cloth. How has the acid affected the color?

Fig. 183.Fig. 183.Strong acids will eat holes like this in cloth.

Action of acids on the nerves of taste.Acids act on the taste nerves in the tongue and give the taste ofsourness; everything sour is an acid. Lemon juice, sour milk, and sour fruits are all too weak acids to injure clothes or skin, but their sour taste is a result of the acid in them acting on the nerves of taste.

Application 79.A girl wanted to make lemonade. She did not know which of two knives to use, a steel-bladed one or a silver-plated one. Which should she have used?Application 80.A woman was going to put up some tomatoes. She needed something large to cook them in. She had a shiny new tin dish pan, an older enamelware dish pan, a galvanized iron water pail, and an old-fashioned copper kettle. Which would have been best for her to use?Make a list of as many acids as you can think of.

Application 79.A girl wanted to make lemonade. She did not know which of two knives to use, a steel-bladed one or a silver-plated one. Which should she have used?

Application 80.A woman was going to put up some tomatoes. She needed something large to cook them in. She had a shiny new tin dish pan, an older enamelware dish pan, a galvanized iron water pail, and an old-fashioned copper kettle. Which would have been best for her to use?

Make a list of as many acids as you can think of.

Explain the following:511. Sugar dissolves readily inhotcoffee.512. The sugar disappears, yet the coffee flavor remains and so does the sweetness of the sugar.513. A tin spoon left overnight in apple sauce becomes black.514. If one's clothes are on fire, rolling over on the ground is better than running.515. Lemon juice bleaches straw hats.516. Will-o'-the-wisps glow at night, deceiving travelers by their resemblance to moving lanterns.517. Tomatoes should never be left in a tin can after it has been opened.518. Boiled milk tastes different from ordinary milk.519. Your hands become very cold after you have washed things in gasoline.520. Wood decays more quickly when wet than when dry.

Explain the following:

511. Sugar dissolves readily inhotcoffee.

512. The sugar disappears, yet the coffee flavor remains and so does the sweetness of the sugar.

513. A tin spoon left overnight in apple sauce becomes black.

514. If one's clothes are on fire, rolling over on the ground is better than running.

515. Lemon juice bleaches straw hats.

516. Will-o'-the-wisps glow at night, deceiving travelers by their resemblance to moving lanterns.

517. Tomatoes should never be left in a tin can after it has been opened.

518. Boiled milk tastes different from ordinary milk.

519. Your hands become very cold after you have washed things in gasoline.

520. Wood decays more quickly when wet than when dry.

Section 55.Bases.

Why does strong soap make your face sting?How is soap made?

Why does strong soap make your face sting?

How is soap made?

"Contains no free alkali," "Will not injure the most delicate of fabrics," "99-44/100% pure,"—such phrases asthese are used in advertising soaps. What is meant by 99-44/100% pure? What is free alkali? Why should any soap injure fabrics? What makes a soap "strong"?

The answer to all these questions is that there are some substances calledbases, which are the opposites of acids, and some of which are as powerful as acids. Lye, ammonia, caustic soda, and baking and washing soda are common bases. The strong bases, like lye and caustic soda, are also calledalkalies. If you want to see what a strong base—an alkali—will do to "the most delicate of fabrics," and to fabrics that are not so delicate, for that matter, try the following experiment:

Experiment 108.To be done by the teacher.If you get any alkali on your skin or clothes, wash it off immediately with vinegar or lemon juice.Put half a teaspoonful of lye and a quarter of a cup of water into a beaker, a small pan, or an evaporating dish. Bring it to agentleboil. Drop a small piece of woolen cloth and a small piece of silk cloth into it and let them boil gently for a couple of minutes. What happens to them? Try a piece of plain cotton cloth, and then a piece of cloth that is mixed wool and cotton or mixed silk and cotton. What happens to them? This is a very good test to determine whether any goods you buy are pure silk or wool, or whether there is a cotton thread mixed with them. Drop one end of a long hair into the hot lye solution. What happens to it? Drop a speck of meat or a piece of finger nail into it.

Experiment 108.To be done by the teacher.If you get any alkali on your skin or clothes, wash it off immediately with vinegar or lemon juice.

Put half a teaspoonful of lye and a quarter of a cup of water into a beaker, a small pan, or an evaporating dish. Bring it to agentleboil. Drop a small piece of woolen cloth and a small piece of silk cloth into it and let them boil gently for a couple of minutes. What happens to them? Try a piece of plain cotton cloth, and then a piece of cloth that is mixed wool and cotton or mixed silk and cotton. What happens to them? This is a very good test to determine whether any goods you buy are pure silk or wool, or whether there is a cotton thread mixed with them. Drop one end of a long hair into the hot lye solution. What happens to it? Drop a speck of meat or a piece of finger nail into it.

From this experiment you can readily see why lye will burn your skin and ruin your clothes. You can also see how it softens the food that sticks to the bottom of the cooking pan and makes the pan easy to clean. Lye is one of the strongest bases or alkalies in the world.

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