50 CHEMICAL PHYSICS. 



bring about a slower molecular motion in the tissues exposed to the 

 cold material. 



The direct transfer of molecular motion is called conduction of heat, 

 but in examining various materials we find that they show a great 

 difference in their power to conduct heat. For instance, if we hold in 

 a flame the end of a glass rod, it may be made red-hot, while but little 

 increase of heat will be perceived at the other end. We accordingly 

 distinguish between good and bad conductors of heat. Gases and 

 liquids (mercury excepted) are bad conductors, and of the solids 

 the metals are the best. The following table gives the comparative 

 heat-conducting power of a number of substances, that of silver being 

 taken as the standard, and represented by 1 : 



Copper . . . .0.96 



Iron . . . - . 0.20 



Stone .... 0.006 

 Water 0.002 



Glass .... 0.0005 



Wool .... 0.00012 



Paper .... 0.000094 



Air . 0.000049 



Convection. On fastening a piece of ice to the bottom of a test- 

 tube, and filling this with water and holding it over a flame in such 

 a manner that only the upper portion of the tube is heated, the water 

 may be made to boil before the ice has been melted. The reason is 

 that water is a bad conductor of heat. If the flame be applied to the 

 lower part of the test-tube, the whole mass of water will remain 

 cold until the ice has melted, and the temperature will then rise 

 evenly through the mass of water, because the heated lighter par- 

 ticles will move upward while the colder ones move downward. 

 Thus ascending and descending currents are produced, equalizing the 

 temperature. The term convection is applied to this method of con- 

 veying and distributing heat. Air or gases behave similarly, and 

 this fact is of practical interest in the construction of chimneys and 

 in heating and ventilating buildings. 



Radiation of heat. A heated body, for instance a ball of red-hot 

 iron, suspended in the air or in a vacuum will heat objects near by. 

 If a screen is placed between these objects and the heated body, no 

 rise in temperature is noticed. Heat is here propagated through 

 space in straight lines, commonly spoken of as heat rays. 



In order to explain these phenomena, as also others closely related 

 to them, such as those of light and electricity, it has been necessary 

 to assume the existence of some agent that serves as a means for this 

 propagation. This hypothetical agent, called ether, is a medium of 



