84 EXPEEIMENTAL GENERAL SCIENCE 



from the warmer body to the cooler one. Heat also travels 

 readily from molecule to molecule through a substance. A 

 piece of metal left with one end in the fire will soon become 

 hot at the other by conduction. Recalling the molecular 

 structure of matter, one easily understands that solids with 

 their molecules close together must in general be the best con- 

 ductors of heat and gases the poorest. All solids, however, do 

 not conduct heat equally well. The metals, especially silver 

 and copper, are among the best conductors, while wood and 

 stone are much poorer. 



77. Radiation. The second way in which the molecular 

 motion of a body may be given to another is by rays sent out 

 from it just as heat rays are sent out from the sun. When we 

 approach the fire, the radiation from it makes us aware that 

 it is giving out heat before we actually come into contact with 

 it. As we have discovered, the heat energy radiated from the 

 sun passes unchanged through most transparent substances, 

 and the heat energy from very hot bodies on the earth act in 

 the same way, but the longer heat rays given off by cooler 

 bodies pass more slowly through even transparent substances. 

 The earth receives and loses heat entirely by radiation. 



78. Insulators. Substances which offer resistance to the 

 passage of heat through them are called insulators. A poor 

 conductor, therefore, is a good insulator, and vice versa. 

 Wood, paper, leather, cotton, wool, feathers, sand, and similar 

 substances make good insulators. Porous materials always 

 make especially good insulators because of the number of 

 small air spaces which they contain. Air is like other gases 

 in being a poor conductor, and, when stationary, as it is in 

 these small spaces, very greatly retards the passage of heat 

 across it. When air can move about, however, it readily 

 carries away heat from a body. 



79. Absorption and Reflection. It has been proved by 

 numerous experiments that the nature of a surface and even 



