163. Heating water makes a steam engine go. 164. When an automobile with good springs and without shock absorbers goes over a rut, the passengers do not get a jolt, but immediately afterward bounce up into the air. 165. Comets swing around close to the sun, then off again into space; how do they get away from the sun? 166. When you wish to pour canned milk out, you need two holes in the can to make it flow evenly. 167. Liquid air changes to ordinary air when it becomes even as warm as a cake of ice. 168. Skid chains tend to keep automobiles from skidding on wet pavement. 169. A warm iron and a blotter will take candle grease out of your clothes. 170. Candies like fudge and nougat become hard and dry when left standing several days open to the air. SECTION 20. _Conduction of heat and convection._ Why does a feather comforter keep you so warm? When you heat one end of a nail, how does the heat get through to the other end? How does a stove make the whole room warm? Here is a way to make heat run a race. See whether the heat that goes through an iron rod will beat the heat that goes through a glass rod, or the other way round: [Illustration: FIG. 56. The metal balls are fastened to the iron and glass rods with drops of wax.] EXPERIMENT 41. Take a solid glass rod and a solid iron rod, each about a quarter inch in diameter and about 6 inches long. With sealing wax or candle grease stick three ball bearings or pieces of lead, all the same size, to each rod, about an inch apart, beginning 2 inches from the end. Hold the rods side by side with their ends in a flame, and watch the balls fall off as the heat comes along through the rods. The heat that first melts off the balls beats. [Illustration: FIG. 57. Does the heat travel faster through the iron or through the glass?] What really happens down among the molecules when the heat travels along the rods is that the molecules near the flame are made to move more quickly; they joggle their neighbors and make them move faster; these joggle the ones next to them, and so on down the line. Heat that travels through things in this way is called _conducted_ heat. Anything like iron, that lets the heat travel through it quickly, is called a _good conductor_ of heat. Anything like glass, that allows the heat to travel through it only with difficulty, is called a _poor conductor_ of heat, or an _insulator_ of heat. A silver spoon used for stirring anything that is cooking gets so hot all the way up the handle that you can hardly hold it, while the handle of a wooden spoon never gets hot. Pancake turners usually have wooden handles. Metals are good conductors of heat; wood is a poor conductor. An even more obvious example of the conducting of heat is seen in a stove lid; your fire is under it, yet the top gets so hot that you can cook on it.