CONDUCTION AND RADIATION 



CLASS NECK 



METAL NECK 



spaces containing air. Since heat crosses stationary air very 

 slowly, most of it remains in the food for a long time and so 

 cooks it. If the tireless cooker be 

 first cooled, it will then keep cold 

 foods cold equally well. In this 

 case the stationary air spaces pre- 

 vent the heat from getting in and 

 warming the food. The thermos 

 bottles which so mysteriously retain 

 the temperature of anything put 

 into them, hot or cold, consist of 

 double- walled vessels with a vacuum 

 between the walls. Heat passes 

 through a vacuum even more 

 slowly than it passes through air, 

 and the interior is prevented from 

 either absorbing or radiating heat 

 rapidly. The warm-blooded 

 animals are kept warm in winter 

 by the stationary air in their fur or 

 feathers. When the weather is ex- 

 tremely cold, both birds and mam- 

 mals are accustomed to fluff up 

 their coats and thus include more 

 air as a greater protection from 

 the cold. The same principle is 

 made use of in winter when we 

 cover the ground about bulbs, 

 half-hardy plants, and 

 planted shrubbery, with a mulch of 

 leaves, straw, or stable manure. While the mulch does not 

 entirely prevent the earth from freezing, the air spaces it con- 

 tains protect the specimens from the sudden changes in tem- 

 perature which are more trying to them than steady cold. 



newly !j, ot , tle - 



* Turton.) 



FIG. 31. Section of a Thermos 

 (Tower, Smith and 



