THE TRANSFERENCE OF HEAT 87 



Experiment 31. Cut off 15 cm. of No. 10 copper and No. 10 

 iron wire and the same length of glass rod of about the same di- 

 ameter. Holding each of these by one end place the opposite end 

 in the flame of a Bunsen burner. Which of the three conducts the 

 heat to the hand first ? 



Experiment 32. Fill a test tube about f full of cold water. Hold- 

 ing the tube by the bottom carefully heat the top part of the water 

 until it boils. Be sure that the flame does not strike 

 the tube above the water, else the tube will break. 

 (Figure 41.) A little piece of ice in the bottom of 

 the test tube makes the action more apparent. A 

 bit of wire gauze or a wire stuffed into the test tube 

 will prevent the ice from coming to the surface. 

 Water conducts heat poorly. The hot water does 

 not sink. Do you conclude that the warm water is heavier or 

 lighter than the colder water ? 



Through solid substances, such as metals, heat travels 

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

 In Experiment 31, 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 transference 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. In 

 transference by conduction each molecule acts as a mes- 

 senger, passing the heat energy on to another that it touches. 

 If two different substances touch each other, the molecules 

 of one substance may conduct heat to the molecules of the 

 other ; but the two substances must be touching each .other 

 or the method of transference cannot be called conduction. 



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



