COLD LIGHT 



35 



spiral in the flame. Could you read by this 

 light? This kind of light is called secondary, 

 because it is caused by the heat of the flame, 

 although the flame gives very little light. First 

 the heat comes from the flame, and secondly 

 the light comes from the wire. 



c. Put a few pieces of wood, or soft coal, in 

 a test tube and hold it, by means of the test- 

 tube holder, in the alcohol or gas flame. Tell 

 what happens. See if you can light what is 

 coming out of the tube. It is gas. A large 

 amount of gas is made by heating coal. 



While the heat from our sources of light is very 

 acceptable during cold weather, it becomes decidedly un- 

 pleasant on the hot nights when we must use lights. 

 For this reason scientists are trying to obtain some 

 source of light which will produce less heat than any we 

 now. use. 



The whiter the light the less heat is produced com- 

 pared with the intensity of the light. Thus reddish or 

 yellowish lights produce much more heat than light, 

 while white light, although it produces a large amount 

 of heat, yields more light in proportion. Name some 

 yellow lights and some white lights. Nature supplies a 

 yellowish light which is practically cold. Fireflies and 

 glowworms are examples of this sort of light. Try to 

 catch some glowworms and examine them. Decaying 

 fishbones sometimes give a faint light, but, like the light 

 obtained from fireflies and glowworms, is of no practical 



