NON-CONDUCTORS OF HEAT. 645 



finally the outer surface heats whatever is in contact with it. 

 Such conveyance of heat from particle to particle, without sen- 

 sible motion, is termed conduction. 



Strictly speaking, according to modern theory, radiant heat 

 is a peculiar kind of undulation communicated to a supposed ex- 

 ceedingly subtile, all-pervading ether ; and conduction is an oscil- 

 lation of the molecules of the conductor itself. But, though we 

 no longer consider heat to be a substance, it is convenient to use 

 the old terms figuratively in describing the phenomena, just as 

 we still say the sun rises and sets, though it is the earth that 

 moves. 



When we sit near an open fire, we are warmed by radiation 

 through the intervening air, while the air itself is heated by con- 

 tact with the fire and passes up the chimney. So radiation and 

 convection, or radiation and conduction, may go on at the same 

 time, and when cooling takes place it is not always easy to tell 

 how much of the effect is due to each of the causes respectively. 

 Hence, substances that are put around hot bodies to retard the 

 change of temperature are often called indiscriminately non-con- 

 ductors, though in fact they may act partly by preventing con- 

 vection or by intercepting radiation. Practically, indeed, it is of 

 little consequence to decide exactly how the loss of heat is pre- 

 vented, but, in the full study of retentive coverings, we must not 

 altogether lose sight of the distinction between mere conduction 

 and general transmission. 



It is a matter of much interest as well as of economical im- 

 portance to find out what substances are most suitable to keep 

 hot bodies warm and cold bodies cool ; and several methods have 

 been devised for making either absolute or comparative trials. 

 After due consideration of the plans used by different experi- 

 menters, the writer has adopted, for the many determinations 

 which he has had occasion to make, an apparatus which may be 

 arranged in three different ways : First, a short, cylindrical me- 

 tallic vessel, with the flat ends vertical, is kept at a constant high 

 temperature by a continual current of steam or hot water passing 

 in at the bottom and out at the top. The non-conductor, of a 

 regular thickness, say one inch, is applied to one of the flat faces 

 of the heater. The other surface of the covering is in contact 

 with a thin brass box, or calorimeter, filled with a known quan- 

 tity of water to receive the transmitted heat. The number of de- 

 grees which the water is raised in an hour gives a definite meas- 

 ure of the amount of heat that the covering allows to pass 

 through. 



Secondly, in trying liquids or air for their conducting power 

 it is desirable to get rid of convection by heating from above, so 

 that the hottest part of the fluid shall be and remain at top. 



