140 



GENERAL SCIENCE 



molecules which move very rapidly are able to get away 

 from the surface of the liquid, it is the slow-moving mole- 

 cules that are left. Temperature depends upon the 

 speed of the molecules in a body. If the swiftly moving 

 molecules are removed, the temperature falls. 



Transference of Heat. Iron is a better conductor of 

 heat than glass. In fact all metallic solids are better con- 

 ductors of heat than non-metallic solids such as glass and 

 wood. 



Experiment 43. Hold a glass rod and an iron rod so that the 

 end of each rod will be in the flame. The other end of the metal 

 rod soon becomes hot, while with the glass rod no change is observed. 



Some Common Substances Arranged in the Order of their 

 Relative Heat Conductivities 



Silver ........ 100 



Copper 74 



Gold 53. 



Brass 25 



Zinc . . . , 19 



Tin . 15 



Iron . . . 

 Lead . . . 

 German silver 

 Ice . . . 

 Glass . . . 

 Hard rubber 



12 



8.5 



6.2 

 .21 

 .05 

 .025 



Experiment 44. Arrange four wires, one of copper, one of 

 brass, one of iron, and one of German silver, on a piece of cardboard 

 as shown in Figure 120. Hold the ends in the flame of a Bunsen 



burner. Determine their relative 

 conductivities by touching a match 

 to them at equal distances from 

 the flame. 



Liquids and gases are poor 

 conductors of heat. That 

 water is a poor conductor 

 may be shown in the follow- 

 ing way : Weight a small piece 

 of ice so that it will rest at 



120. Metals vary m their 



conduction of heat. the bottom OI a test tUD6 full 



FIG. 



