162 EXPERIMENTAL GENERAL SCIENCE 



infra-red rays which the eye fails utterly to note, but which 

 when properly handled, will affect the photographic plate. 

 In this region of the spectrum the majority of the heat rays 

 also occur. Beyond the violet end of the spectrum are the 

 ultra-violet rays. Some of these vibrate 1,500,000,000,000,000 

 times a second. These rays kill germs, cause skin to tan and 

 freckle, and may even be used for making photographs, 

 though most of the rays which make ordinary photographs 

 are found in the violet end of the spectrum. Fine particles 

 of dust, water, or ice may act like a prism in breaking up light 

 into these primary colors, or they may even stop some of the 

 colors and allow the others to come to us. To such causes many 

 colors of the sky and clouds are due. 



135. Absorption and Reflection. Colored bodies have no 

 colors of their own. We call them colored only when they 

 have the power to reflect or absorb some of the light rays 

 falling upon them. A piece of red glass for instance is red 

 because it absorbs or stops all the rays of light except those 

 we call red, allowing the latter to pass. A red apple, however, 

 is red for a different reason. In this case, it sorts out and 

 reflects red rays and absorbs all the others. When a body 

 absorbs all the light rays, it will of course give back none, and 

 we call it black. If an apple be placed in light which has no 

 red rays in it, there will be none to reflect, and it will conse- 

 quently appear black. Placing a green object in red light 

 would have the same effect. The energy of the colors absorbed 

 is changed to heat which gives reason for the statement that 

 black clothing is warmer than white. Red, being nearer the 

 warm end of the spectrum, is properly called a warm color, 

 while blue and violet are known as cold colors. White, as 

 we have seen, is a mixture of all the colors. 



136. Fluorescence. Some bodies have the power to change 

 the color of the light falling upon them and are said to be 

 fluorescent. In fluorescence, as in phosphorescence, the rays 



