The electricity you have been reading about and experimenting with in this section is called _static electricity_. "Static" means standing still. The electricity you rubbed up to the surface of the comb or glass stayed still until it jumped to the bit of paper or hair; then it stayed still on that. This was the only kind of electricity most people knew anything about until the nineteenth century; and it is not of any great use. Electricity must be flowing through things to do work. That is why people could not invent electric cars and electric lights and telephones before they knew how to make electricity flow steadily rather than just to stand still on one thing until it jumped across to another and stood there. In the next chapter we shall take up the ways in which electrons are made to flow and to do work. _APPLICATION 48._ Explain why the stroking of a cat's back will sometimes cause sparks and make the cat's hairs stand apart; why combing sometimes makes your hairs fly apart. Both of these effects are best secured on a dry day, because on a damp day the water particles in the air will let the electrons pass to them as fast as they are rubbed up to the surface of the hair. INFERENCE EXERCISE Explain the following: 291. If you shuffle your feet on a carpet in clear, cold weather and then touch a person's nose or ear, a slight spark passes from your finger and stings him. 292. If you stay out in the cold long, you get chilled through. 293. The air and earth in a greenhouse are warmed by the sun through the glass even when it is cold outside and when the glass itself remains cold. 294. When you hold a blade of grass taut between your thumbs and blow on it, you get a noise. 295. Shadows are usually black. 296. Some women keep magnets with which to find lost needles. 297. You can grasp objects much more firmly with pliers than with your fingers. 298. If the glass in a mirror is uneven, the image of your face is unnatural. 299. A sweater clings close to your body. 300. Kitchens, bathrooms, and hospitals should have painted walls. CHAPTER EIGHT ELECTRICITY SECTION 33. _Making electricity flow._ What causes a battery to produce electricity? What makes electricity come into our houses?