and has a glossy, silky appearance. Cotton thus treated is called 66 mercerized" cotton.

182. How Are Silk and Wool Obtained? - Silk and wool are obtained from animals. Silk-growing is one of the greatest industries of Japan and China. The eggs of the silkworm are allowed to hatch in long trays. These trays are filled with leaves and the tiny "worms" feed upon these leaves until they become large, fat, white creatures. There are usually so many of these worms feeding in a room that you can hear them crunch their food.

When the silkworm is grown, it spins a cocoon, as our common caterpillars do, and prepares to come out a beautiful, winged creature. But after the spinning is completed, men place the cocoon in hot water to kill the silkworm; then they unwind the tiny thread which forms the cocoon. Many of these threads are woven together to give us a little piece of silk cloth. Silk fibers are long, smooth, beautiful, and very strong.

Wool is obtained by shearing sheep; this is done in the spring time. The heavy, wool coat grows in the autumn to protect the animal from winter's cold and will drop off when warm weather comes, if it is not cut. The fibers of wool are short and thick, with projections that overlap one another (Fig. 105).

Unlike vegetable fibers, animal fibers are destroyed by bases, and are not readily acted upon by acids. No method of washing wool should be used which will force the cells closer together, or the wool will shrink and finally become stiff and board-like.

183. How Is Clothing Washed? - When fibers are woven into cloth, tiny spaces (pores) are left between the threads of the cloth. These pores form an absorbing surface for the skin (cf. § 108). Perspiration and the waste it removes from the body, as well as dead skin

FIG. 106. The equipment of a modern laundry. Name the different machines.

itself, are all caught in the surface of the cloth. Clothing that has been worn too long has a damp, sticky feel because its pores are filled. This clothing should not be worn again until it has been washed. The reason why clean garments "feel so good," as we say, is because they have a fresh absorbing surface.

In ancient times the women took the soiled clothing of the family to a stream and rubbed it out upon the rocks. Later the washing was done at home with the aid of hot water and soap. Now we have stationary tubs, washing machines, wringers, and even machines run by electricity (Fig. 106). So our washing is done better and done more easily all the time, as we recognize the importance of clean clothing. What would you think of the practice, common in some countries, of sewing on heavy clothing in the fall and leaving it on all winter?

184. How Does Soap "Work"? Why do we use soap? We say that it removes dirt. If you think of it, you will see that the dirt we want taken away from clothing is usually some form of grease or some form of soot. When, therefore, the soap solution is rubbed on greasy cloth, it breaks up the grease into tiny droplets and surrounds them. Thus they are separated from the cloth and can be washed away by the water. Water alone cannot do this.

If you try the action of soap upon some kerosene, you can understand its cleaning power for grease. Into a test tube or small-mouth bottle put some kerosene and water and shake the two together. The kerosene will break up into droplets, but when you stop shaking, the droplets will soon unite and the layer of kerosene will float upon the layer of water. But if you shake the kerosene with a dilute soap solution, the kerosene does not separate again into a layer by itself; it remains as tiny droplets mixed with the soap solution. In the same way, the butter fat of milk is kept suspended in tiny droplets by something in the milk. A mixture like this is called an emulsion.

When a soap solution is rubbed upon soot, the tiny particles of soot are surrounded by the soap solution and so taken off the clothing.

How do you suppose people ever learned to use soap? 185. How Is Soap Made? In primitive times soapmaking was carried on by every family (Fig. 107). In the spring all the winter's wood ashes were pounded down in a barrel and water was allowed to soak through the ashes and caught when it came out at the bottom. This water was boiled down to down to form homemade lye (potash). Later, lye was bought instead of being made. Then the lye, either homemade or commercial, was placed in a huge kettle

with melted fat (from FIG. 107. The making of soap at home the winter's supply of

meat) and the two were cooked together in the open. Sometimes the cooking was continued for two or three days. Then the "soft soap " was cooled and put away in barrels for the next year's use.

Nowadays soap is made on a large scale in factories (Fig. 108). Nearly every meat-packing house has a soap factory as a part of its business. Why? When the meats are cut and packed, there are always fat portions that cannot be sold. These are used to make soap. Some soaps are made of vegetable fats, such as olive oil and

Soap is made by boiling the
The lye "cuts
"the fat and

cottonseed oil, instead of animal fats. fat or oil with sodium hydroxide (lye). it disappears into solution. Common table salt is then added to the solution, and the soap is "salted out." It floats on the top, is skimmed off, pressed, and cut into cakes. Sometimes a longer process is used, so that glycerine can be made at the same time. What sort of substance is glycerine? What are some of its uses?

FIG. 108.

How soap is made on a large scale. Such kettles are three stories high and hold enough material to make about 700,000 bars of soap. 186. What is Dry Cleaning? Certain materials, such as silks, woolens, and kid gloves, lose their soft finish if they are washed often, as cotton and linen goods are. For this reason a dry cleaning shop is now found in even our small cities. Dry cleaning is done by means of gasoline instead of water. The gasoline used for the cleaning evaporates quickly from the material, leaving it clean and soft and as pretty as when new. People