WHY GLOWWORMS GLOW. When a glowworm glows at night, or when the head of a match glows as you rub it on your wet hand in the dark, we call the light _phosphorescence_. The name "phosphorus" means light-bearing, and anything like the element phosphorus, that glows without actively burning, is said to be phosphorescent. Match heads have phosphorus in them. Phosphorescence is almost always caused by chemical change. The energy released is a dim light, not heat or electricity. Sometimes millions of microscopic sea animals make the sea water in warm regions phosphorescent. They, like fireflies, glowworms, and will-o'-the-wisps, have in them some substance that is slowly changing chemically, and energy is released in the form of dim light as the change takes place. Most luminous paint is phosphorescent for the same reason,--there is a chemical change going on that releases energy in the form of light. When you poured the hydrochloric acid on the zinc to make hydrogen, the flask became warm; the chemical change going on in the flask released heat energy. _APPLICATION 76._ Explain why pouring cold water on cold quicklime makes the slaked lime that results boiling hot; why a cat's eyes shine in the dark; why a piece of carbon and a piece of zinc placed in a solution of sal ammoniac will make electricity run through the wire that connects them; why fire is hot. INFERENCE EXERCISE Explain the following: 481. A baking potato sometimes bursts in the oven. 482. Turpentine is used in mixing paint. 483. Sodium is a metal; chlorine is a poisonous gas; yet salt, which is made up of these two, is a harmless food. 484. When bricklayers mix water with cement and lime, the resulting mortar boils and steams. 485. Green plants will not grow in the dark. 486. Parts of the body are constantly uniting with oxygen. This keeps the body warm. 487. Water will not always put out a kerosene fire. 488. Delicately colored fabrics should be hung in the shade to dry. 489. A match glows when you rub it in the dark. 490. Candy hardens when it cools. SECTION 52. _Explosions._ What makes a gun shoot? What makes an automobile go?