RADIATION OF HEAT. 27 



the temperature of the surrounding air. All bodies, no matter 

 what their temperature may be, constantly radiate heat ; the 

 loss thus entailed being more or less made up by absorption 

 of radiated heat from other bodies, and in some cases by 

 conduction of heat from bodies in contact with them. The 

 rays of the sun are composed chiefly of rays of light, and, to a 

 smaller extent, of rays of heat ; the former being capable of 

 becoming converted into the latter and vice versa. If, during 

 summer in a hot country, we ascend a mountain, say to a 

 height of 16,000 ft., we shall find that the rays of the sun are 

 as powerful as in the plains, but that the temperature of the 

 air is below freezing point. We may therefore conclude in 

 such a case, that the heat of the air in the plains is due to 

 contact with the ground, buildings, and other objects which 

 have become heated by the rays of the sun ; but not to 

 radiation. Heat received by means of radiation is partly 

 absorbed and partly reflected. Thus, when a horse is placed in 

 a cold stall, his body by conduction (p. 30) raises the tem- 

 perature of the air which surrounds it, and radiates heat that 

 warms the partitions, walls, etc. upon which it falls. These 

 objects in their turn warm the air with which they are in 

 contact, and radiate heat to other objects, including the horse, 

 until finally the temperature of the atmosphere of the stall 

 becomes more or less raised, and more or less equalised. In 

 the open, almost all the heat radiated by a horse is lost, as far 

 as he is concerned, in his surroundings, and consequently 

 during cold weather the temperature outside is lower than 

 in the stable. 



The light rays of the sun (luminous rays) are capable of 

 easily going through ordinary window-glass, which offers a 

 considerable obstacle to the passage of the heat rays 

 (obscure rays). Consequently, when sunshine falls on the 

 panes of a closed stable window, the heat rays are more or 

 less taken up by the glass, and the light rays enter the 



