524. When you stub your toe while walking, you tend to fall forward. 525. Electric lamps glow when you turn on the switch. 526. If you use much ammonia in washing clothes or cleaning, your hands become harsh and dry. 527. If a person swallows lye or caustic soda, he should immediately drink as much vegetable oil or animal oil as possible. 528. Water is made of hydrogen and oxygen; air is made of nitrogen and oxygen; yet while things will not burn in water, they will burn easily in air. 529. The backs of books that have been kept in cases for several years are not as bright colored as the side covers. 530. If you try to burn a book or magazine in a grate, only the outer pages and edges burn. SECTION 56. _Neutralization._ When you put soda in vinegar, what makes the vinegar less sour? When we use sour milk for cooking, why does the food not taste sour? One of the most interesting and important facts about acids and bases is that if they are put together in the right proportions they turn to salt and water. Strong hydrochloric acid (HCl), for instance, will attack the skin and clothes, as you know; if you should drink it, it would kill you. Caustic soda (NaOH), a kind of lye, is such a strong alkali that it would dissolve the skin of your mouth in the way that lye dissolved hair in Experiment 108. Yet if you put these two strongly poisonous chemicals together, they promptly turn to ordinary table salt (NaCl) and water (H_2O). Or, as the chemists write it: NaOH+HCl -> NaCl+H_2O. You can make this happen yourself in the following experiment, using the acid and base dilute enough so that they will not hurt you: EXPERIMENT 111. Although strong hydrochloric acid and strong caustic soda are dangerous, if they are diluted with enough water they are perfectly harmless. You will find two bottles, one labeled "_caustic soda_ (NaOH) diluted for tasting," and the other labeled "_hydrochloric acid_ (HCl) diluted for tasting." From one bottle take a little in the medicine dropper and let a drop fall on your tongue. Taste the contents of the other bottle in the same way. _It is not usually safe to taste things in the laboratory. Taste only those things which are marked "for tasting."_ Now put a teaspoonful of the same hydrochloric acid into a clean evaporating dish. Lay a piece of litmus paper in the bottom of the dish. With a medicine dropper gradually add the dilute caustic soda (NaOH), stirring as you add it. Watch the litmus paper. When the litmus paper begins to turn blue, add the dilute caustic soda drop by drop until the litmus paper stays blue when you stir the mixture. Now add a drop or two more of the acid until the litmus turns pink again. Taste the mixture.