The strong bases all have a bitter taste even when dissolved in a great deal of water. If there is not much water mixed with the bases, their solutions will destroy skin and flesh and the mucous membrane that lines the mouth. Hence we must use a great deal of care in handling lye.

178. How Can We Test for Bases and Acids? How do bases differ from acids? If you had a solution which looked like water, and yet you knew that it might contain an acid or a base, how would you find out which it was? The easiest way is to use litmus. Litmus is a coloring matter obtained from lichens, a kind of plant. An acid will turn blue litmus to red. A base will turn red litmus to blue. Litmus may be used in the form of a solution and a few drops of it may be poured into the liquid you are testing; or it may be in the form of colored paper and a slip of the paper may be dipped into the liquid. If you test orange juice, what happens to your litmus? Test some of the things you find at home to see which show an acid test and which show a basic test. Also bury a piece for an hour or two in some moist soil in a flower pot and see whether the soil is acid or not. Farmers use this test, because acid soil is not good for the raising of most crops. Purple cabbage solution may be used in exactly the same way as litmus solution. 179. How Does an Acid Act with a Base? - We can now learn something still more interesting about acids and bases. Dip your thumb and forefinger into a solution of caustic lye and then rub them together. They feel slippery, or slimy, do they not, as though they were covered with strong soap solution? Now add to the

lye solution some vinegar, or lemon juice, a little at a time. You will soon find that the solution no longer has the slimy feel it had at first. The acid of the vinegar or lemon juice has destroyed this power of the lye. chemist says the acid has neutralized the lye.

The

Put a piece of red litmus paper into some fresh caustic lye solution; what color does the litmus have now? Then add some hydrochloric acid solution, little by little, to the lye solution, stirring as you do so. What color does the litmus take on? The acid has neutralized, or destroyed, the power of the lye to turn litmus blue.

If there is no base left in the lye solution to which the hydrochloric acid was added, what do you suppose is left? Put the solution into a saucer or into an evaporating dish, and drive off the water by heating the dish gently. What is left in the dish? Taste it; it is common salt. So salt was formed when caustic lye was neutralized by hydrochloric acid. When vinegar or lemon juice neutralizes the caustic lye solution, there is another substance formed which is not common salt, but is like it in some ways. The chemist calls all such substances salts, since they are formed by the neutralization of a base by an acid, as common salt is.

Salts are very common. Marble, limestone, table salt, blue vitriol, saltpeter, and alum are all called salts by the chemist.

180. Exercises. 1. Galvanized iron is iron covered with zinc. Is it a safe material for dishes in which acid fruits are cooked?

2. The acid present in canned tomatoes causes milk to become

curdy. In making cream tomato soup some baking soda is added to the tomato to prevent the curding; explain how it acts.

3. The spots of limestone on a glass water pitcher are easily removed by a little vinegar or lemon juice. Can you tell why?

4. Collect some wood ashes, cover them with water, and after the ashes have settled, pour off the water. Then test the water with pink and blue litmus paper. Does the solution have an acid or a basic action? Could you use wood ashes to “sweeten " a "sour" soil?

5. Test solutions of alum, washing soda, and common salt separately with pieces of red and blue litmus. How does each behave?

6. To a lump of old mortar in a dish add a few drops of a dilute acid. Describe what happens. Can you prove that the gas formed is carbon dioxide? What is left undissolved? What gas does fresh mortar take from the air as it hardens, or sets?

CHAPTER XIX

WASHING AND CLEANING

181. What are the Materials of Clothing? - In very ancient times savage men kept warm by clothing themselves with the skins of animals; even today furs are

sometimes used for warmth as well as for decoration. Still, as you know, the common materials for our cloth

ing are woven fabrics; they come from both animal and vegetable sources. Silk and wool are from animals; while the vegetable fibers which can be woven to advantage are cotton and flax. What an interesting story it would be, if we could learn how men first came to use these materials for

Men gather this covering from the plant, separate it from its seeds, and use the fibers in making cotton cloth. The fibers are hollow tubes, flat and twisted.

FIG. 105.

Silkworm's

Cocoon

Silk Fibers

The fibers out of which fabrics are woven, and the silkworm's cocoon. Note the overlapping scales of wool fibers.

The stalks of the flax plant are used to make linen cloth. These fibers are hollow, like those of cotton, but straight instead of being twisted. They have thick walls with central openings. Linen cloth is stronger than cotton, but cotton is the lighter and more elastic.

Cotton and linen are easily destroyed by strong acids, but are not easily harmed by bases. In fact, if cotton is soaked in a strong base for a short time, and is then washed thoroughly, it is made stronger