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 analysis. 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, butter, eggs, rice, oatmeal, cornmeal, onions. [Illustration: 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 glass. Does it turn milky? Pump ordinary air through it with a bicycle pump. Now blow air from your lungs through a glass tube into some fresh limewater until it turns milky. By this test you can always tell if carbon dioxid (CO_2) is present. [Illustration: FIG. 189. The limewater test shows that there is carbon dioxid in the air.]