COMMUNICATION OF HEAT, 



225 



reflected by it in parallel lines to the surface of the opposite mirror, N, where 

 they will be caused to converge to its focus, B, and ignite the powder or 

 phosphorus at that point. 



Fig. 201. 



512. Polished metallic surfaces constitute 

 reflectors of the bcst reflcctors of heat ; but all bright and 

 light colored surfaces are adapted for this pur- 

 pose to a greater or less degree.* 



Water requires a longer time to become hot in a bright tin vessel than in a 

 dark colored one, because the heat is reflected from the bright surface, and 

 does not enter the vessel 



How does the ^l^- Thc powcr of absorbing heat varies 

 6orT.ing°*^heat ^^^^ almost cvery form of matter. Surfaces 

 ^^'T? are good absorbers of heat in proportion as 



they are poor reflectors. The best radiators of heat also 

 are the most powerful absorbers, and the most imperfect 

 reflectors. 



Dark colors absorb heat from the sun more abundantly than light ones. 

 Tliis may be proved by placing a piece of black and a piece of white cloth 

 upon the snow exposed to the sun ; in a few hotirs the black cloth will havo 

 melted the snow beneath it, while the white cloth wiU have produced little 

 or no effect upon it. 



The darker any color is, the warmer it is, because it is a better absorbent 

 of heat. The order may be thus arranged : 1 , black (warmest of all) ; 2, violet ; 

 3, indigo ; 4, blue ; 5, green ; 6, red ; 7, yellow ; and 8, white (coldest of all). 



• Of 100 rays falling at an angle of G0° from the perpendicular, polished gold will reflect 

 76 .• silver 62 • brass, 62; brass \rithout polish. 52 ; polished brass varnished. 41 ; looking- 

 glass, 20 ; glass plate blackened on the back, 12 ; and metal plate blackened, 6. 



10* 



