Jnuc 6, 1 872 J 



NATURE 



107 



clifTerence of emissive power of the radiator at different tempera- 

 tures. It was stated in the preceding article, (vol. v. pp. 505-507), 

 that the radiant power of one square foot of cast-iron develops 

 o'oSo thermal unit per minute for each degree of differential 

 temperature at 65", and 0^337 unit at 3,000° ; hence that the 



emissive power is increased ^„ = 4'2I times for an incre- 



o oSo 

 ment of 3,000 - 65 = 2,935°. Kxperiments conducted in the 



Fl^. /. 



mean time show that the radiant power of one squire foot of cast 

 iron maintained at a differential temperature of i.Soo° is 335 

 units per minute, hence that the emissive power at this stage of 



incandescence amounts to ^^^_^^ = 0'iS6 unit for each degree of 



I, boo 

 differential temperature. Our investigations have thus proved 

 that at 65° the emissive power is 0080 thermal unit, at 1,800° 



0'i86 unit, and at 3,000° 0'337 for each degree of differential 

 temperature. We have accordingly establislied the fact that the 



emissive power increases nearly in the same ratio as the in- 

 tensities, being fully quadrupled between the differential tempera- 



