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, face down. 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.

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.

[Ill.u.s.tration: FIGS. 174 and 175. Where the negative is dark, the print is light.]

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 parts _is_ affected 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 (H_2O) from the earth and carbon dioxid (CO_2) 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 next day 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 gla.s.s of water that contains iodine.

The iodine will color the leaf dark where the cells contain starch. (See Experiment 115, page 373.) 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 by _combining_ oxygen with the hydrogen of our food (forming water) and by combining oxygen with the carbon in our food (forming carbon 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 (CO_2).

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 (CO_2) and water (H_2O) 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 carbon dioxid 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.

INFERENCE EXERCISE

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?

You have already done an experiment showing that electricity can start chemical change, for you changed water into hydrogen and oxygen by pa.s.sing 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 gla.s.s 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.

Now fasten a fine bare wire 5 or 6 inches long around a small piece of copper, and another like it around a piece of nickel, 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.

[Ill.u.s.tration: FIG. 176. The copper and the nickel cube ready to hang in the cleansing solution.]

[Ill.u.s.tration: FIG. 177. Cleaning the copper in acids.]

Pour over the piece of copper enough of the cleansing solution to cover it.[9] _The 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 the _negative_ wire in the green nickel solution. You can tell if you have it on the negative wire, for in that case bubbles will rise from it during 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 the _positive_ wire 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 gla.s.s or china cups, and the hydrochloric acid must be measured in a silver-plated spoon or in gla.s.s--not in tin.]

[Ill.u.s.tration: 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 copper still 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.