72 HEAT OR CALORIC. 



where it is clean, and especially if glazed, by frozen rain'} Because 

 snow is a good reflector, and dirt, from its rough dark surface, absorbs 

 heat rapidly. 



(kk.) Why on copper plates painted black, white, gray, fyc. does 

 wax melt soonest on the black and other dark colors, and scarcely at 

 all on the white* when they are exposed to the sun *? The answer 

 is founded on the general effect of colors on the absorption and radi- 

 ation of heat. 



(II.) Why do pieces of cloth of different colors, black, white, and 

 intermediate shades, when laid on snow in the sunshine, sink into the 

 snow very differently, the black deepest, and the white not at all ? 

 The answer is the same as under kk. 



(mm.) Why in summer, is the temperature of the earth several de- 

 grees lower than that of the air, especially in a clear night ? It is 

 owing chiefly to radiation, as beautifully illustrated by Dr. Wells. 



(nn.) Why, in hot weather, is a house cooler if kept dark, than if 

 light and air are freely admitted ? Because the radiant heat, flow- 

 ing, not from the sun only, but from all external objects, some of 

 which are often much heated, is also excluded. 



(oo.) Why is white a good color for the roof of an ice house, and 

 black a bad color for any roof? Because the former reflects, and 

 the latter absorbs the heat rapidly. 



EXPERIMENTAL ILLUSTRATIONS. 



i. Inequality of conducting power. Dr. Hare, from I to 11, ex- 

 cept 3, 4, and 8. 



" Let there be four rods, severally of metal, 

 wood, glass, whale bone, each cemented at one 

 end to a ball of sealing wax. Let each rod, at 

 the end which is not cemented to the wax, be 

 successively exposed to the flame excited by a 

 blow pipe. It will be found, that the metal be- 

 comes quickly heated throughout, so as to fall 

 off" from the wax but, that the wood, or whale- 

 bone, may be destroyed, and the glass bent, by 

 the ignition, very near to the wax, without melt- 

 ing it, so as to liberate them." 



* The colored surfaces receiving the rays, and the waxed side being downwards. 



