30 



RADIATION OF HEAT. 



to receive it, as represented in the annexed figure. The diffe- 

 rential thermometer answered admirably for this purpose, as 

 from its con- 

 struction it is un- 

 affected by the 

 temperature of 

 the room, while 

 the slightest 

 change in the 

 temperature of 

 the focal spot is 

 immediately in- 

 dicated by it. 



Two metallic mirrors were occasionally used in conducting 

 these experiments. The mirrors being arranged so as to stand 

 exactly opposite 

 to each other, 

 with their prin- 

 cipal axes in the 

 same line ; when 

 a lighted lamp or 

 hot canister is 

 placed in the fo- 

 cus of one mir- 

 ror, the incident rays are reflected by that mirror against the 

 other, and collected in its focus. 



The following table exhibits the relative radiating power of 

 various substances, with which the surface of the canister was 

 coated, as indicated by the effect upon the differential thermo- 

 meter : 



Lamp black . .100 



Water by estimate . 100 + 



Writing-paper . 98 



Sealing-wax . 95 



Crown glass * 90 



Plumbago . . 75 



It thus appears that lamp black radiates five times more of the 

 heat of boiling water than clean lead, and eight times more than 

 bright tin. The metals have the lowest radiating power, which 

 arises from their brightness and smoothness. If allowed to 

 tarnish, their radiating power is greatly increased. Thus the 



Tarnished lead 

 Clean lead 

 Iron, polished 

 Tin plate, gold, 

 copper 



silver, 



45 

 19 

 15 



12 



