40 HEAT. 



gives out heat,- in fact, more than twice as fast, the ra- 

 pidity of radiation being more than in proportion to the 

 temperature. 



79. POLISH is UNFAVORABLE TO RADIA- 



What are the . 



effects of rough TioN. A coffee-pot oi well brightened 



metal will keep its contents hot much bet-. 

 diation? t er than a dingy, blackened one, thus re- 



warding the housewife for her pains. The brightness 

 is not the cause of this effect. It is owing to the fact 

 that polished surfaces are more dense, and dense sur- 

 faces do not allow heat to pass readily. But if the pol- 

 ished coffee-pot be covered with muslin so as to give it 

 a less dense surface, radiation and consequent cooling 

 will proceed more rapidly again. One would think 

 that the polished surface beneath the cloth would have 

 the same effect in retaining the heat as before, and that 

 the cloth would still further retard its escape ; but ex- 

 periment proves that this is not the case. Radiation 

 depends on the surface, without regard to what is be- 

 neath it, and the superiority of the cloth as a radiator 

 is more than sufficient to make up for its non-conduct- 

 ing influence. Rough uncompact surfaces, generally, 

 radiate well. High polish being unfavorable to radia- 

 tion, stoves should not be too highly polished. The 

 high polish of soldiers' helmets makes them much 

 cooler than if they were made of dull metal. 

 ,. r , . 80. COLOR DOES NOT AFFECT RADIATION.-^ 



What effect 



has color on A black coat wastes no more of the heat 



of the body by radiation than a white one. 



Except in the direct rays of the sun, one is just as 



warm as the other. But the former absorbs and imparts 



