Fire and Heat 109 



and the objects on the earth is absorbed and converted into 

 heat so long as the vibrations do not exceed four hundred 

 billion a second. When the rate is between four hundred billion 

 and eight hundred billion vibrations a second and these fall 

 upon the eye, the organ of sight, they are converted into light. 

 Both heat and light must therefore be considered as effects 

 of radiant energy. 



Other effects may be mentioned. Radiant energy, when it 

 falls upon certain substances, induces in them chemical changes 

 and is in this respect known as chemical energy. This effect 

 is familiar in photography, for it is this sort of energy which 

 produces the changes on the exposed plate or film in the camera. 

 In other substances or conditions it causes electrical changes, 

 and is described as electrical energy. Because of the different 

 effects which the sun's energy produces in different kin.ds or 

 qualities of bodies, it can be correctly spoken of, not as heat or 

 light, but as radiant energy. 



Color and surface in absorption and radiation. Besides the 

 different heat capacities of familiar substances, common ex- 

 perience furnishes many instances where the color and the char- 

 acter of the surface of objects affect the capacity for absorption 

 of heat energy. Snow sprinkled with soot or ashes melts more 

 rapidly than snow which is clean and white. A square of 

 black cloth and another of white cloth of the same material 

 and size, spread over snow so as to receive equally the sun's 

 rays, show very different results in a short time. Under the 

 white square the snow melts very slowly, and more rapidly 

 under the black, which seems to mean that the black absorbs 

 and transmits more heat energy than the white. The im- 

 portance of color in heat absorption and radiation or trans- 

 mission may be shown in several ways. 



Exercise : Radiating surfaces. Get two bright tin cans of equal 

 capacity, which may be closed tightly with stoppers. Fit into each 

 stopper a thermometer. Blacken one can with lampblack by holding 

 it over a kerosene or gas flame. Fill both cans with water of the same 



