ABSORPTION OF HEAT 29 



person is to be in the sunshine. For the same reason, dark- 

 colored earth absorbs more heat than the lighter-colored soil, 

 which may be desirable when heat is needed for crops. 



If materials were transparent, they would not absorb any 

 heat. There is, however, no substance that is truly transpar- 

 ent, and so there is always some absorption. The atmosphere 

 absorbs about forty per cent of the sun's heat which reaches 

 it, and dirty water absorbs practically all of the heat within a 

 few inches of its surface. If we sit by a window, in the sun- 

 shine, we may be warmed by the energy from the sun, although 

 the window glass, through which the energy passes, is not 

 noticeably heated. Therefore radiant " heat " is really not 

 heat, but a form of energy which may be changed into heat. 



An application of absorption of heat is the solar heater for 

 water. This consists of a large number of pipes arranged in a 

 glass-covered box, which is usually placed upon the roof of the 

 house. The water must pass through all of the pipes before 

 it can reach the outlet. The interior of the box and also the 

 pipes are painted black, without any gloss, and the water 

 absorbs enough heat to be sufficiently warm for washing 

 dishes and for other domestic purposes. Why is the box 

 covered with glass? 



References : 



1. 1103 : 27-28. Absorption of Heat by the Atmosphere. 



2. 1803 : 464-465. Radiation and Absorption. 



a. 1801 : 292-295. Absorption Selective Absorption. 



6. 1802 : 324. Effect of Absorption. 



c. 1804 : 422. Good Radiators, Good Absorbers. 



d. 1805 : 328-331. Absorption Selective Absorption. 



e. 1806 : 392. Absorbing Power. 



/. 1807 : 170-172. Reflection and Absorption at Surfaces. 

 g. 1808 : 221-223. Radiating and Absorbing Powers. 

 h. 1810 : 177. Absorption of Radiant Energy. 



