THE ENERGY OF THE SUN 



143 



panied by heat. If we hold a sensitive thermometer in the 

 violet end of the spectrum so that the violet rays fall upon the 

 bulb, the reading of the mercury will be practically the same as 

 when the thermometer is held in any dark part of the room ; if, 

 however, the thermometer is slowly moved toward the red end 

 of the spectrum, a change occurs and the mercury rises slowly 

 but steadily, showing that heat rays are present at the red 

 end of the spectrum. This agrees with the popular notion, 

 formed independently of science, which calls the reds the 

 warm colors. Every one of us associates red with warmth ; 

 in the summer red is rarely worn, it looks hot; but in winter 

 red is one of the most pleasing colors because of the sense of 

 warmth and cheer it brings. 



All light rays are accompanied by a small amount of heat ', 

 but tJie red rays carry the most. 



What seems perhaps the most unexpected thing, is that 

 the temperature, as indicated by a sen- 

 sitive thermometer, continues to rise if 

 the thermometer is moved just beyond 

 the red light of the spectrum. There 

 actually seems to be more heat beyond 

 the red than in the red, but if the 

 thermometer is moved too far away, 

 the temperature again falls. Later 

 we shall see what this means. 



138. The Energy of the Sun. It 

 is difficult to tell how much of the 

 energy of the sun is light and how 



much is heat, but it is easy to deter- FlG 89 ._ The energy of the 

 mine the combined effect of heat and 

 light. 



Suppose we allow the sun's rays to fall perpendicularly 

 upon a metal cylinder coated with lampblack and filled with 



sun can be measured in heat 

 units. 



