RADIATION. 45 



90. ABSENCE OF DEW ON POLISHED SUR- 



"Why is dew 



not ' deposited FACES. Dew does not form on polished 



surfaces because they are poor radiators, or, 

 in other words, do not allow their heat to 

 escap'e, and thereby produce the degree of cold which 

 is required to form dew. Leaves and grass receive 

 most dew, because they are the best radiators. 



91. SUPPOSED RADIATION OF COLD. 



Why does the . 



thermometer li a piece oi ice be held before a ther- 

 broucht "near mometer > ^ wn "l cause the mercury to sink. 

 ice? It is not because cold has been radiated 



from the ice, but because the thermometer, in common 

 with all other bodies, is constantly giving out heat, 

 and when the ice is near, it does not get its due portion 

 in return. The ice cuts off the heat that would have 

 come to it from other objects behind it, and gives it but 

 little in its place. 



92. REFRACTION OF HEAT. Rays of 



How arc rays 



of heat re- heat from the sun and other objects, are 

 refracted or bent out of their course, on 



passing from one medium to 



another, similarly to rays of 



light. By ordinary glass 



prisms most of the heat rays 



are refracted in a less degree. 



93. HEAT RAYS AND CHEMICAL RAYS. 

 The light which proceeds from the sun, is 



rays and che- accompanied by rays of heat and others 



mical rays ? . 



called chemical or actinic rays. In the 

 analysis of light by a prism, the chemical rays accu- 

 mulate principally in the region of the violet color of 



