36 LIGHT 



value. This kind of light is called phosphorescence. So 

 far. man has not produced any really cold light which is 

 sufficiently strong for practical purposes. The experi- 

 ment shows one form of cold light which is used to a 

 slight extent on match-boxes, clock faces, and around 

 door bells so that they may be seen in the dark. 



Experiment 20. Cold Light. 



Apparatus: A matchbox with the word "matches" 

 made of luminous paint. 



a. Place the matchbox so that the direct sunlight 

 will fall upon it for at least ten minutes, and then exam- 

 ine it in a dark room. It may be necessary to remain 

 in the dark room for a few minutes before your eyes 

 will become accustomed to the darkness. What do yon 

 see? 



b. Keep the matchbox in a perfectly dark place for 

 two or three days and then examine it without removing 

 it from the dark. Can you see anything? 



c. Place the matchbox in the sunlight for ten min- 

 utes and again examine in a dark room. What do you 

 conclude is the real source of light in the case of lumi- 

 nous paints? 



Review Questions, 9. 



1. Which side of trees has the most moss? Why? 



2. How can you locate the south by means of a 

 watch? 



3. If the sun rises exactly in the east, what is the 

 time of sunset? 



4. Could you tell time by the Great Dipper? 



