146 



ELEMENTARY GENERAL SCIENCE 



CHAP. 



Such conduction causes no visible movement of the parts of the 

 body. This is the process by which heat passes through solids. 

 Those substances which easily transmit heat in this way are 



called good conductors, while 

 those which offer a considerable 

 amount of resistance to its pass- 

 age are called bad conductors. 



EXPT. 140. Wrap a piece 

 of paper smoothly round a 

 brass tube and hold in the 

 flame of a gas burner. The 

 paper is not scorched. Wrap 

 the paper around a wooden 

 rod of the same size, and 

 heat as before : the paper is 

 scorched, Fig. 62. Brass is a 

 good conductor, wood but a 

 poor one. How does this ex- 

 plain what you have noticed ? 

 EXPT. 141. Twist an iron and a copper wire together atone 



end. At about four inches from the joint fasten a marble 



on each with beeswax. Heat the joint in a Bunsen flame. 



Notice that the marble on the copper is the first to fall. Why ? 

 EXPT. 142. Make a short coil of stout copper wire inch 



internal diameter. Pass it over the wick of a candle without 



Fig. 62. Experiment 140. 



FIG. 63. Experiment 143. 



touching the wick. The candle is extinguished owing to 

 the cooling effect of the wire which conducts away the heat. 



EXPT. 143. Turn on, but do not light, a gas jet. Hold over 

 it a wire gauze, and light the gas above the gauze. Notice 

 that the flame does not strike through. Why? Vary the 



