386. Linemen working with live wires wear heavy rubber gloves. 387. Crayon will not write on the smooth, glazed parts of a blackboard. 388. Varnish and shellac may be thinned with alcohol. 389. Filtering will take mud out of water, but it will not remove salt. 390. Explain why only one wire is needed to telegraph between two stations. SECTION 42. _Crystals._ How is rock candy made? Why is there sugar around the mouth of a syrup jug? How are jewels formed in the earth? You can learn how crystals are formed--and many gems and rock candy and the sugar on a syrup jug are all crystals--by making some. Try this experiment: EXPERIMENT 83. Fill a test tube one fourth full of powdered alum; cover the alum with boiling water; hold the tube over a flame so that the mixture will boil gently; and slowly add boiling-hot water until all of the alum is dissolved. Do not add any more water than you have to, and keep stirring the alum with a glass rod while you are adding the water. Pour half of the solution into another test tube for the next experiment. Hang a string in the first test tube so that it touches the bottom of the tube. Set it aside to cool, uncovered. The next day examine the string and the bottom of the tube. EXPERIMENT 84. While the solution of alum in the second test tube (Experiment 83) is still hot, hold the tube in a pan of cold water and shake or stir it until it cools. When white specks appear in the clear solution, pour off as much of the clear part of the liquid as you can; then pour a little of the rest on a glass slide, and examine the specks under a microscope. [Illustration: FIG. 148. Alum crystals.] In both of the above experiments, the hot water was able to dissolve more of the alum than the cold water could possibly hold. So when the water cooled it could no longer hold the alum in solution. Therefore part of the alum turned to solid particles. When the string was in the cooling liquid, it attracted the particles of alum as they crystallized out of the solution. The force of adhesion drew the near-by molecules to the string, then these drew the next, and these drew more, and so on until the crystals were formed. But when you kept stirring the liquid while it cooled, the crystals never had time to grow large before they were jostled around to some other part of the liquid or were broken by your stirring rod. Therefore they were small instead of large. Stirring or shaking a solution will always make crystals form more quickly, but it will also make them smaller.