This is a regular chemical test for certain elements when they are combined with oxygen. The cobalt will always change the borax bead to the blue you got; the chromium will make the bead emerald green or, in certain kinds of flame, ruby red; etc. If you wanted to know whether or not certain substances contained cobalt combined with oxygen, you could really find out by taking a grain on a borax bead and seeing if it turned blue.

THE HYDROCHLORIC ACID TEST FOR SILVER. The experiment in which you tested the action of light in effecting chemical change, and in which you made a white powder or precipitate in a silver nitrate solution by adding hydrochloric acid (page 327), is a regular chemical test to find out whether or not a thing has silver in it. If any silver is dissolved in nitric acid, you will get a precipitate (powder) when hydrochloric acid is added. Make the test in the following experiment:

EXPERIMENT 114. _Use distilled water all through this experiment if possible._ First wash two test tubes and an evaporating dish thoroughly, rinsing them several times. Into one test tube pour some nitric acid diluted 1 to 4. Heat this to boiling, then add a few drops of hydrochloric acid diluted 1 to 10. Does anything happen? Pour out this acid and rinse the dish thoroughly. Now put a piece of silver or anything partly made of silver into the bottom of the evaporating dish.

Do not use anything for the appearance of which you care.

Cover the silver with some of the dilute nitric acid, put the dish over the Bunsen burner on a wire gauze, and bring the acid to a gentle boil. As soon as it boils, take the dish off, pour some clean, cold water into it to stop the action, and pour the liquid off into the clean test tube. Add a few drops of the dilute hydrochloric acid to the liquid in the test tube. What happens? What does this show must have been in the liquid?

You can detect very small amounts of silver in a liquid by this test.

It is a regular test in chemical a.n.a.lysis.

THE IODINE TEST FOR STARCH. A very simple test for starch, but one that is thoroughly reliable, is the following:

EXPERIMENT 115. Mix a little starch with water. Add a drop of iodine. What color does the starch turn? Repeat with sugar.

You can tell what foods have starch in them by testing them with iodine. If they turn black, blue, or purple instead of brown, you may be sure there is starch in them. And if they do not turn black, blue, or purple, you can be equally sure that they have no starch in them. Some baking powders contain starch to keep them dry. Test the baking powder in the laboratory for starch. Often a little cornstarch is mixed with powdered sugar to keep it from lumping. Test the powdered sugar in the laboratory to see if it contains starch.

Test the following or any other ten foods to see if any of them are partly made of starch: salt, potatoes, milk, meat, sausage, b.u.t.ter, eggs, rice, oatmeal, cornmeal, onions.

[Ill.u.s.tration: FIG. 188. The white powder that is forming is a silver salt.]

THE LIMEWATER TEST FOR CARBON DIOXID. In crowded and badly ventilated rooms carbon dioxid in unusual amounts is in the air. It can be detected by the limewater test.

EXPERIMENT 116. Pour an inch or two of limewater into a gla.s.s. Does it turn milky? Pump ordinary air through it with a bicycle pump. Now blow air from your lungs through a gla.s.s tube into some fresh limewater until it turns milky. By this test you can always tell if carbon dioxid (CO_2) is present.

[Ill.u.s.tration: FIG. 189. The limewater test shows that there is carbon dioxid in the air.]

Carbon dioxid turns limewater milky as it combines with the lime in the limewater to make tiny particles (a precipitate) of limestone. If you pour seltzer water or soda pop into limewater, you get the same milkiness, for the bubbles of carbon dioxid in the charged water act as the carbon dioxid in your breath did. If you pumped enough air through the limewater you would produce some milkiness in it, for there is always some carbon dioxid in the air.

The purpose of these experiments is only to give you a general notion of how a chemist a.n.a.lyzes things,--by putting an unknown substance through a series of tests he can tell just what that substance contains; and by accurately weighing and measuring everything he puts in and everything he gets out, he can determine how much of each thing is present in the compound or mixture. To learn to do this accurately takes years of training. But the men who go through this training and a.n.a.lyze substances for us are among the most useful members of the human race.

INFERENCE EXERCISE

Explain the following:

551. A little soda used in canning an acid fruit will save sugar.

552. The fats you eat are mostly digested in the small intestine, where there is a large excess of alkali.

553. The dissolved food in the liquid part of the blood gets out of the blood vessels and in among the cells of the body, and it is finally taken into the cells through their walls.

554. Ammonia takes the color out of delicate fabrics.

555. Dishes in which cheese has been cooked can be cleaned quickly by boiling vinegar in them.

556. Prepared pancake flour contains baking powder. It keeps indefinitely when dry, but if the box gets wet, it spoils.

557. When water or milk is added to prepared pancake flour to make a batter, bubbles appear all through it.

558. When a roof leaks a _little_, a _large_ spot appears on the ceiling.

559. Gasoline burns quietly enough in a stove, but if a spark gets into a can containing gasoline vapor, there is a violent explosion.

560. Turpentine will remove fresh paint.

GENERAL REVIEW INFERENCE EXERCISE

Explain the following:

561. We can remove fresh stains by pouring boiling water through them.

562. A ship can be more heavily laden in salt water than in fresh water.

563. Water flies off a wet dog when he shakes himself.

564. In cooking mola.s.ses candy, baking soda is often added to make it lighter.

565. An egg will not stand on end.

566. Women who carry bundles on their heads stand up very straight.

567. To get all crayon marks off a blackboard, the janitor uses _vinegar_ in water.

568. Sunlight makes your skin darker.

569. Water puts out a fire.

570. You get a much worse shock from a live wire when your hands are wet than when they are dry.

571. Stone or brick buildings are cool in summer but warm in winter.

572. If you take the handle off a faucet, it is almost impossible to turn the valve with your fingers.

573. Sparks fly from a grindstone when you are sharpening a knife.

574. Violin strings are spoiled by getting wet.

575. The oxygen of the air gets into the blood from the lungs, although there are no holes from the blood vessels into the lungs.

576. You push a b.u.t.ton or turn a key switch and an electric lamp lights.

577. A rubber comb, rubbed on a piece of wool cloth, will attract bits of paper to it.

578. People whose eyes no longer adjust themselves have to have "reading gla.s.ses" and "distance gla.s.ses" to see clearly.

579. When you look through a triangular gla.s.s prism, things appear to be where they are not.

580. Lye and hot water poured down a clogged kitchen drainpipe clear out the grease.