RADIATION OF HRAT. 



425 



moisture in the morning. If the temperature of the external air be at or be- j 

 low the freezrng point, this deposition will form a rough coating of ice on the < 

 pane. , Let a small piece of tin-foil be fixed on a part of the exterior surface j 

 of one pane of the window in the evening, and let another piece of tin-foil be ( 

 iixed on a part of the interior surface of another pane. In the morning it will j 

 be found that that part of the interior surface which is opposite to the external i 

 foil will be nearly free from ice, while every other part of the same pane will ] 

 be thickly covered with it. On the contrary, it will be found that the surface i 

 of the internal tin-foil will be more thickly covered with ice than any other part ' 

 of the glass. These effects are easily explained by the principle of radiation. < 

 When the tin-foil is placed on the exterior surface, it reflects the heat which 

 strikes on the exterior surface, and protects that part of the glass which is 

 covered from its action. The heat radiated from the objects in the room 

 striking on the surface of the glass, penetrates it, and encountering the tin-foil 

 attached to the exterior surface, is reflected by it through the dimensions of 

 the glass, and its escape into the exterior atmosphere is intercepted ; the por- 

 tion of the glass, therefore, covered by the tin-foil, is in this case subject to 

 the action of the heat radiated from the chamber, but protected from the action 

 of the external heat. The temperature of that part of the glass is therefore 

 less depressed by the effects of the external atmosphere than the temperature 

 of those parts which are not covered by the tin-foil. Now, glass being, as 

 will appear hereafter, a bad conductor of heat, the temperature of that part op- 

 posite to the tin-foil does not immediately affect the remainder of the pane, 

 and consequently we find that while the remainder of the interior surface of 

 the pane is thickly covered with ice, the portion opposite the tin-foil is com- 

 paratively free from it. On the contrary, when the tin-foil is placed on the 

 internal surface, it reflects powerfully the heat radiated from the objects in 

 the room, while it admits through the dimensions of the glass, the heat pro- 

 ceeding from the external atmosphere. The portion of the glass, therefore, 

 covered by the tin-foil, becomes colder than any other part of the pane, and 

 the tin-foil itself receives the same temperature, which is not reduced by the 

 effect of the radiation of objects in the room, because the tin-foil itself is a good 

 reflector of heat, and a bad absorber. Hence the tin-foil presents a colder sur- 

 face to the atmosphere of the room than any other part of the surface of the 

 pane, and consequently receives a more abundant deposition of ice. 



If a body, which is a good radiator of heat, be exposed in a situation where 

 other good radiators are not present, it will have a tendency to fall in its tempe- 

 rature below the temperature of the surrounding medium ; because, in this case, 

 while it loses heat by its own radiation, its absorbing power is not satisfied by 

 a corresponding supply of heat from other objects. A clear sky, in the ab- 

 sence of the sun, has scarcely any sensible radiation of heat ; if, therefore, a 

 good radiator be exposed to the aspect of an unclouded firmament at night, it 

 will lose heat considerably by its own radiation, and will receive no corres- 

 ponding portion from the radiation of the firmament to repair this loss, and its 

 temperature consequently will fall. 



A curious experiment made by Dufay affords a striking illustration of this 

 fact. He exposed a glass cup, placed in a silver basin, to the atmosphere du- 

 ring a cold night, and he found in the morning a copious deposition of moisture 

 on the glass, while the silver vessel remained perfectly dry. He next reversed 

 the experiment, and exposed a silver cup in a glass basin. The result was the 

 same : the glass was still covered with moisture, and the metal free from it. 

 Now metal is a bad radiator of heat, and consequently has a tendency to pre- 

 serve its temperature. Glass is a much better radiator, and has therefore a 

 | tendency to lose its temperature. These vessels being exposed to the aspect 



