302 RADIANT HEAT. 



The difference was greatest in still air, and diminished with the 

 violence of its motion : 



Time of cooling. 



Wind. Bright. Blackened. 



Gentle 44 minutes 35 minutes. 



Strong 23 " 20£ " 



Violent,. 9£ " 9 



Hence the effect is different from conduction b) r air. 



3.) The most exact series of experiments was that in which he used 

 conjugate reflectors, a differential thermometer having one bulb in the 

 focus, and a cubical tin canister of hot water, (the temperature of 

 which was seen by the projecting stem of a thermometer,) and each 

 side of which could be coated with a different substance, and pre- 

 sented successively towards the reflector. 



The following results collected together afford the best view of the 

 general nature of the conclusions relative to the influence of the 

 state of the surface on the radiation of heat. — (Inquiry, pp. 81, 90, 

 110.)' 



Lampblack 100* 



Water, (estimated).. 100 



Writing paper 98* 



Rosin • • • • 96 



Sealingwax. 95 



Crown glass • • • • 90 



China ink . • • • • 88* 



Ice .•••• 85 



Minium . • » 80* 



Isinglass 80 



Plumbago 75 



Thick film of oil 59 



Film of jelly 54t 



Thinner film of oil 5 If 



Tarnished lead. 45 



Film of jelly, (^ of former quantity) 38 



Tin scratched with sandpaper 22 



Mercury 20 



Clean lead 19 



Polished iron ^- 15 



Polished tin, gold, silver, copper 12 



Thin lamina of gold, silver, or copper leaf on glass 12^: 



* From comparing the results marked, it appears that the effect 

 follows no relation to color. Softness probably tends to increase radia- 

 tion. 



t Thickness of film increased beyond a certain limit does not in- 

 crease the radiation. 



X The tenuity is not sufficient to produce any diminution of effect, 

 which probably would take place if thinner films could be applied. 



