424 RADIATION OF HEAT. 



Numerous facts of ordinary occurrence, and many interesting natural phe- 

 nomena, admit of easy and satisfactory explanation on the principle of the 

 above theory of radiation. 



Vessels intended to contain a liquid at a higher temperature than the sur- 

 rounding medium, and to keep that liquid as long as possible at the higher 

 temperature should be constructed of materials which are the worst radiators 

 of heat. Thus, tea-urns and tea-pots are not adapted for their purpose when 

 constructed of black porcelain. A black porcelain tea-pot is the worst con- 

 ceivable material for that vessel, for both its material and color are good ra- 

 diators of heat, and the liquid contained in it cools with the greatest possible 

 rapidity. On the other hand, a bright metal tea-pot is best adapted for the 

 purpose, because it is the worst radiator of heat, and therefore cools as slowly 

 as possible. A polished silver or brass tea-urn is better adapted to retain the 

 heat of the water than one of a dull brown color, such as is most commonly 

 used in England. 



A tin kettle retains the heat of water boiled in it more effectually if it is 

 kept clean and polished, than if it be allowed to collect the smoke and soot, to 

 which it is exposed from the action of the fire. When coated with this, its 

 surface becomes rough and black, and is a powerful radiator of heat. 



A set of polished fire-irons may remain for a long time in front of a hot fire 

 without receiving from it any increase of temperature beyond that of the cham- 

 ber, because the heat radiated by the fire is all reflected by the polished sur- 

 face of the irons, and none of it is absorbed ; but if a set of rough, unpolished 

 irons, were similarly placed, they would speedily become hot, so that they 

 could not be used without inconvenience. The polish of fire-irons is ; there- 

 fore, not merely a matter of ornament, but of use and convenience. The rough, 

 unpolished poker, sometimes used in a kitchen, soon becomes so hot that it 

 cannot be held without pain. 



A close stove, intended to warm an apartment, should not have a polished 

 surface, for in that case it is one of the worst radiators of heat, and nothing 

 could be contrived more unfit for the purpose to which it is applied. On the 

 other hand, a rough unpolished surface of cast-iron is favorable to radiation, 

 and a fire in such a stove will always produce a more powerful effect. 



A metal helmet and cuiras, worn by some regiments of cavalry, is a 

 cooler dress than might be at first imagined. The polished metal being a 

 nearly perfect reflector of heat, throws off the rays of the sun, and is incapable 

 of being raised to an inconvenient temperature. Its temperature is much less 

 increased by the influence of the sun than that of common clothing. 



The polished surfaces of different parts of the steam-engine, especially of the 

 cylinder, is not matter of mere ornament, but of essential utility. A rough 

 metal surface would be a much better radiator of heat than the polished sur- 

 face, and if rust were collected on it, its radiating power would be still further 

 increased, and the steam contained in it would be more exposed to condensa- 

 tion by loss of heat. 



It may be frequently observed that a deposition of moisture has taken place 

 on the interior surface of the panes of glass of a chamber-window, on a morn- 

 ing which succeeds a cold night. The temperature of the external air during 

 the night being colder than the atmosphere of the chamber, it communicates 

 its temperature to the external surface of the glass, and this is transmitted to 

 the interior surface, which is exposed to the atmosphere of the room. This 

 atmosphere is always more or less charged with vapor, and the cold of the ex- 

 ternal surface of the glass acting on the air in contact with it, reduces its tem- 

 perature below the point of saturation, arid a condensation of vapor takes place 

 on the surface of the panes, which is observed by a copious deposition of 



