102 Our Surroundings 



another object; by convection, that is, by movement of currents 

 in gases and liquids ; and by conduction, that is, by transmission 

 of heat from one part of a body to another part or from one 

 body to another by direct contact. 



Radiation. If your feet are held in front of the burning 

 wood in a fireplace they become warm. Heat passes out in all 

 directions from the fire without the help of surrounding objects. 

 It reaches your feet without any visible means. This method of 

 heat movement is called radiation. Heat radiates from its source 

 in straight lines and in all directions. 



The sun's heat, called radiant heat, reaches the earth by radia- 

 tion. Some investigators claim that it travels as fast as light, 

 186,000 miles per second. It is a curious fact that the heat from 

 the sun will pass through transparent material, such as glass, with- 

 out heating it. This may be shown by feeling the glass of a win- 

 dow when the sun is shining brightly on it. 



The florist takes advantage of this fact by growing his plants 

 in a glass-covered house, called a greenhouse. The rays of the 

 sun travel through the panes of glass, and the objects inside are 

 thus warmed. The glass will feel cold. 



Radiant heat in its passage through the air from the sun to 

 the earth does not heat the air, as is proved by the fact that 

 the upper layers of the atmosphere are cold. The lower layers 

 are not heated by these rays as is often supposed. They derive 

 their heat by coming in contact with the -heated surface of the 

 earth. 



The amount of heat that any body throws off by radiation 

 depends on its substance, its~ surf ace, its color and its size. As 

 a rule a substance that absorbs heat readily radiates it readily. 

 A rough surface radiates more Heat than a smooth surface. Hence 

 radiator surfaces often have a rough finish. A black object 

 radiates more heat than one of any other color. This is one 

 reason why stoves are usually black. A large surface radiates 

 more heat than a small one. For this reason, stove pokers often 

 have coiled wire handles, instead of solid handles, affording more 

 radiating surface and therefore giving off much of their heat 

 before it reaches the hand. 



