222 WELLS'S NATUKAL PniLOSOPHY. 



parts, and the heat is communicated by the motion of the 

 particles among each other. 



These currents take place so rapidly, that if a thermometer be placed at 

 the bottom and another at the top of a long jar, the fire being applied below, 

 the upper one will begin to rise almost as soon as the lower one. The 

 movement of the particles of water in boiling will be understood by reference 

 to Fig. 200. They may be rendered visible by adding to a flask of boiling 

 water a few small particles of bituminous coal, or flowers of sulphur. 



Air and other gases are heated in precisely the same manner aa wateij 

 and this method of communicating heat is termed convection. 



Heat, however, passes by conduction between the particles of both liquids 

 and gases, but to such a slight extent, that they were for a long time re- 

 garded as entirely incapable of conducting heat. 



In what man- ^^^- Tlic proccss of cooHng in a h'quid is 

 cooild? "'^"''^ directly -the reverse of that of heating. The 

 particles at the surface, by contact with the 

 air, readily lose their heat, become heavier, and sink, while 

 the warmer particles below in turn rise to the surface. 



To heat a liquid, therefore, the heat should be applied at the bottom of 

 the mass ; to cool it, the cold should be applied at the top, or surface. 



The facility with which a liquid may be heated or cooled depends in a great 

 degree on the mobility of its particles. "Water may be made to retain its 

 heat for a long time by adding to it a small quantity of starch, the particles 

 of which, by their viscidity or tenacity, prevent the free circulation of tho 

 heated particles of water. For the same reason soup retains its heat longer 

 than water, and all thick liquids, like oil, molasses, tar, etc., require a consid- 

 erable time for cooling. 



p 504. "When the hand is placed near a hot body suspended 



piienomena of in the air, a sensation of warmth is perceived, even for a 

 radiation, considerable distance. If the hand be held beneath the body, 



the sensation will be as great as upon the sides, although the heat has to 

 shoot down through an opposing current of air approacliing it. This effect 

 does not arise from the heat being conveyed by means of a hot current, since 

 all the heated particles have a uniform tendency to rise ; neither can it de- 

 pend upon the conducting power of the air, because aeriform substances pos- 

 sess that power in a very low degree, while the sensation in the present case 

 is excited almost on the instant. This method of distributing heat, to did" 

 tinguish it from heat passing by conduction, or convection, is called radiation, 

 and heat thus distributed is termed radiant, or radiated heat. 



T^ .. V :.. 505. All bodies radiate heat in some meas- 



Do all bodies 



radiate heat urc, but uot all CQuallv well ; radiation bemar 



squally well ? ' ii ^i t- 



m proportion to the roughness of the radiatmg 

 surface. All dull and dark substances are, for the most 



