9 8 GENERAL SCIENCE 



conduct its heat to the hand. The difference in the rate 

 of conductivity of substances can be also experienced by 

 stepping with bare feet from a woolen carpet on to an 

 oilcloth in a cool room. Linen clothing is a better con- 

 ductor of heat than woolen. At what time of the year 

 should woolen and at what time should linen be worn? 

 Why? 



The influence that the air in the meshes of clothing 

 has upon its power to conduct heat is very great. Felt, 

 feathers, and fur make very warm coverings because 

 they are very poor conductors of heat and thus prevent 

 the escape of heat from the body. Their great number of 

 minute spaces containing air or other gases helps to make 

 them poor conductors of heat. Freshly fallen snow is a 

 great protection to such crops as wheat, rye, and hay. 



The roots of fruit trees are 

 also protected by having snow 

 on the ground during very 

 cold weather, as the ground 

 does not freeze very deep 

 under the snow. 



(c) Convection is the process 

 of transferring heat by the 

 flow of liquids or gases. This 

 process can be illustrated by 

 placing a few crystals of potas- 

 sium permanganate in a flask 



almost full of clear water and 

 CONVECTION CURRENTS then glowly heat j ng u Qver ft 



Showing currents while water is -r> -u T-U i 



being heated. Bunsen burner. The coloring 



matter will soon reveal currents 



of water moving around in the flask, going upward just 

 above the flame and downward on the sides of the flask. 



