HOW ROCK CANDY IS MADE. Rock candy is made by hanging a string in a strong sugar solution or syrup and letting the water evaporate slowly until there is not enough water to hold all the sugar in solution. Then the sugar crystals gather slowly around the string, forming the large, clear pieces of rock candy. The sugar around the mouth of a syrup jug is formed in the same way. You always get crystallization when you make a liquid too cool to hold the solid thing in solution, or when you evaporate so much of the liquid that there is not enough left to keep the solid thing dissolved. When you make fudge, the sugar forms small crystals as the liquid cools. When a boat has been on the ocean, salt crystals form on the sails when the spray that has wet them evaporates. But crystals may form also in the air. There is always some moisture in the air, and when it becomes very cold, some of this moisture forms crystals of ice. If they form up in the clouds, they fall as snow. If they form around blades of grass or on the sidewalk, as the alum crystals formed on the string, we have frost. Still another place that crystals occur is in the earth. When the rocks in the earth were hot enough to be melted and then began to cool, certain substances in the rocks crystallized. Some of these crystals that are especially hard and clear constitute precious and semi-precious stones. _APPLICATION 64._ Explain why you beat fudge as it cools; why the paper around butter becomes encrusted with salt if it is exposed to the air for some time. INFERENCE EXERCISE Explain the following: 391. Dynamos have _copper_ brushes to lead the current from the coils of wire to the line wires. 392. A megaphone makes the voice carry farther than usual. 393. Copper wire is used to conduct electricity, although iron wire costs much less. 394. A flute gives notes that differ in pitch according to the stops that are opened. 395. There are usually solid pieces of sugar around the mouth of a syrup jar. 396. You can beat eggs quickly with a Dover egg beater. 397. When ocean water stands in shallow open tanks for some time, salt begins to form before the water has all evaporated. 398. In a coffee percolator the boiling water goes up through a tube. As this water drips back through the ground coffee beans, it becomes brown and flavored, and the coffee is made. 399. Kerosene will clean off the rim of soap and grease that forms in bathtubs. 400. Beating cake frosting or candy causes it to sugar. SECTION 43. _Diffusion._ How does food get into the blood? Why can you so quickly smell gas that is escaping at the opposite side of a room?