TRANSFERENCE OF HEAT 53 



Without the heat of the sun there would be no life upon 

 the earth, no flowing streams, no changing winds, none of 

 the restless energy which makes the world as we know it. 

 It is therefore essential to understand how heat is trans- 

 ferred from one place to another. 



Through solid substances, such as metals, heat travels 

 quite readily, through others such as glass, less rapidly. 

 In Experiment 23, we found that heat traveled along 

 some rods faster than it did along others. In no case, 

 however, was there any indication that there was a trans- 

 ference of the particles composing the rods. In the boiling 

 of the water at the top of the test tube, there was no 

 indication that the water particles moved to the bottom of 

 the tube. In these cases, the heat is simply transferred 

 from molecule to molecule. 



This kind of heat transference is called conduction. 

 Conductors may be good or bad, as was shown by the dif- 

 ferent materials used in the experiments. We use iron 

 for our radiators so that the heat of the steam may readily 

 be given out to the room, and we cover our steam pipes 

 with asbestos when we wish to retain the heat, 

 because asbestos is a poor conductor and will 

 keep the heat in the pipes. 



Experiment 25. Hold a piece of burning paper 

 under a bell jar held mouth downward. Notice the 

 air currents as indicated by the smoke. Paper soaked 

 in a moderately strong solution of saltpeter and dried 

 burns with a very smoky flame. Fig- 28. 



Experiment 26. Fill a 500 cc. round-bottomed flask half full of 

 water and place on a ring stand above a Bunsen burner. Stir in a 

 little sawdust. Some of it should fall to the bottom of the flask. 

 Gently heat the bottom of the flask. Notice the currents. 



When the water was heated at the bottom of the flask 

 and when the burning paper was held under the bell glass, 



