22 EXPERIMENTAL GENERAL SCIENCE 



energy, which travels about 186,000 miles a second and takes 

 some eight minutes to cross the more than 92,000,000 miles 

 that separate us from the sun. Radiant energy moves for- 

 ward in straight lines, but the waves of which it consists 

 vibrate very rapidly back and forth across the direction of the 

 rays. It is not until it comes into contact with some form of 

 matter that radiant energy becomes heat or light. The prin- 

 cipal difference between them appears to be that light rays 

 are much more rapid than heat rays, and affect a different one 

 of the senses. Light rays vibrate with inconceivable rapidity. 

 The rays that give the sensation of red vibrate 392,000,000,- 

 000,000 times a second and those which give the sensation of 

 violet vibrate 757,000,000,000,000 times a second. Some of 

 the invisible rays which affect the photographer's plates and 

 films vibrate twice as fast as the violet rays (134). Both 

 heat and light rays can be bent or reflected, very smooth sur- 

 faces, such as mirrors, being most effective in this respect. 

 As radiant energy, heat rays readily pass through transparent 

 substances, but when they strike opaque bodies some of the 

 energy is absorbed as heat. When the heat is again turned 

 to radiant energy of a longer wave length (77), it does not 

 then pass easily through even transparent bodies. It is due 

 to this fact that greenhouses and hot-beds are sometimes called 

 " traps to catch sunbeams." Most of the radiant energy 

 readily passes in through the glass roof, but after being changed 

 to the longer heat rays, finds escape difficult, and thus remains 

 to warm the plants in the enclosure. The water vapor in the 

 air acts in a similar way to keep the earth warm, allowing the 

 radiant energy to pass through it, but preventing the passage 

 of the long heat rays. 



Practical Exercises 



1. Why does the snow melt at the base of trees and other objects 

 before it melts in open fields? 



