The nitric acid did to the copper in this experiment exactly what the hydrochloric acid did to the zinc shavings when 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 called _copper 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? [Illustration: 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 of sourness; 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?