RELATION OF HEAT AND LIGHT. 



191 



on ; similar reasoning being applied to all other shades of color. If a body 

 appears perfectly black, it absorbs all the sun's light, and reflects none ; if it 

 be perfectly white, it reflects all the sun's light and absorbs none ; but perfect 

 colors, whether black or white, or of whatever other tint they may be, do not 

 exist in nature. No body exhibits an absolute black or an absolute white, how- 

 ever near these limits they may approach. 



If an opaque body of any color be exposed to the direct rays of the sun, it 

 will be observed to rise in its temperature, or become warm. If it be of a 

 black color, it will exhibit a rapid and considerable increase of temperature. 

 Next to black, a body of a blue color will absorb most heat ; next follow green, 

 yellow, and red, and white least of all. 



That black should absord most heat, and white least, follows immediately 

 from the fact that a body of a black color absorbs nearly all the solar rays, and 

 with them their heat ; while a body of a white color reflects nearly all the rays, 

 and with them reflects their heat. Of all the constituent parts of solar light, 

 that which possesses the least heating power is the blue light. A body, therefore, 

 which reflects this only, must absorb all the most powerful heating rays ; and hence 

 we see why an opaque object of a blue color receives the most heat, next to black. 

 The green light has a certain heating power, less than the red or yellow, but 

 more than the blue. A body, therefore, which reflects the green light, absorb- 

 ing the others, reflects more heat than a blue or Mack object, but less than 

 objects of those colors which occupy the lower part of the prismatic spectrum. 

 Such a body, therefore, receives less heat from the solar light than those of a 

 darker shade, and more than those of a lighter. The application of the same 

 reasoning will explain why bodies of a yellow or red color absorb still less 

 heat. 



If several pieces of cloth, of the same size and quality, but of different col- 

 ors — black, blue, green, yellow, and white — be thrown on the surface of snow 

 in clear daylight, but especially in sunshine, it will be found that the black 

 cloth will quickly melt the snow beneath it and sink downward. The blue 

 will do the same, but less rapidly ; the green still less so ; the yellow slightly, 

 and the white not at all. These effects illustrate the principle just explained. 

 We see. also, that the warmth or coolness of clothing depends as well on its 

 color as its quality. A white dress, or one of a light color, will always be 

 cooler than one of the same quality of a dark color, and especially so in clear 

 weather, when there is much sunshine. A white, or light color, reflects heat 

 copiously, and absorbs little ; while a black and dark color absorbs copiously 

 and reflects little. From this we see that experience* has supplied the place of 

 science in directing the choice of clothing. The use of light colors always 

 prevails in summer, and that of dark colors in winter. 



Of transparent objects, some, such as air and the gases, are almost perfectly 

 so, transmitting nearly all the light to which they are exposed. Such bodies 

 are, consequently, invisible ; since the light which passes through them, and 

 which alone can affect the sight, suffers no effect different from that which it 

 would undergo if they were not present, and if the space through which it 

 passed were an absolute vacuum. Such bodies, since they arrest no portion of 

 the light in its progress, receive no heat from it. The same is true of some 

 liquids, as pure water ; and of some solids, though in a less degree, as plate 

 glass. The rays of solar light, passing through a pane of plate glass, produce 

 little effect on its temperature ; but some little effect is produced, since no glass, 

 however pure, is perfectly transparent ; but even were it admitted that glass 

 and other transparent bodies were absolutely transparent to all the luminous 

 rays of solar light, it might happen that they would absorb those invisible cal- 

 orific rays which are proved to exist in it, and to be less refrangible than any 



