CHAPTER IX* 

 LIGHT 



129. Sources of Light. People are now living who can 

 remember when candles and whale-oil lamps were the ordi- 

 nary means of lighting houses and streets. Later kerosene 

 oil and gas came into use, and now electricity is very generally 

 used for lighting streets, public places, and houses. Light 

 from gas, candles, and oil lamps is caused by particles of 

 carbon made red-hot in the flame. The heat of the flame 

 is due to combustion of gas and vapors from the heated 

 wax or oil. Thus the origin of the light is the heat of 

 combustion. In the case of an electric lamp, the conductor 

 is heated to a glowing heat or incandescence by the resis- 

 tance which is offered to the current. 



Besides combustion and electricity, there is another source 

 of light: friction between two bodies. Here it is resistance 

 to motion which produces heat. Often a small particle of 

 metal or stone is heated to light-giving temperature, as when 

 a spark is struck by a horse's shoe on the pavement. Meteors 

 are made red-hot by the resistance of the air, through which 

 they pass with immense velocity. 



130. Intensity of Light. The rapid vibration of mole- 

 cules, from whatever cause, produces heat; and if the vibra- 

 tions are sufficiently rapid, heat causes the body to become 

 incandescent. The brightness or intensity of light increases 

 with the temperature of the luminous body. The cause of 

 the vibrations is not always known. This is the case with 

 the sun and the stars. 



In stating the amount of light given from a luminous body, 

 the term candle power is used. One candle power is the 



120 



