dl4 RADIANT HEAT. 



taken up by M. De la Roche, whose researches are justly entitled to 

 the high celebrity they have acquired. The report "of the French 

 Institute upon them will be found in the Annals of Phil., 0. S., ii, 

 161; and a full account of the experiments in Biot's Traite de Phys., 

 iv, 640. 



The whole series of results is as follows: 



Source of heat. 



RISK OF THERMOMETER IN 1 MIN. CENT1G. 



No screen. 



Transparent 

 screen. 



Blackened 

 screen. 



1. Vessel of mercury, temp. 180° cent 



2. Vessel of mercury, boiling, 346° 



3. Iron, 427° 



4. Copper, 960°, (1) 



5. Ditto (2) 



6. Argand lamp — no chimney 



7. Argand lamp — chimney 



o 

 3.94 

 1G. 33 

 32.8 

 38.97 

 71.54 

 21. 12 

 23.44 



0.22 



1.36 



4.70 



11.83 



21.41 



7.29 



12.82 



o 



0.07 



0. 17 



0.31 



0.40 



0.73 



0.21 



0.23 



The first two experiments of this series have been already consid- 

 ered. The third, or iron, at 427° centig. was at a red heat, its 

 temperature of luminosity in the dark being about 400°. This, there- 

 fore, and the subsequent part of the series are affected by the con- 

 sideration that light was emitted, which materially alters the case, as 

 we shall presently observe. 



De la Roche infers from these experiments that a portion of simple 

 radiant heat is transmitted directly in the way of radiation through 

 glass, and that this increases as the temperature is raised. 



A thick glass, though very transparent, stops heat more than a 

 thin glass less so; the difference is less as the temperature is raised. 



A portion of the heat having been intercepted by one screen, a 

 proportionally much less diminution is caused by the introduction of 

 a second; hence he infers that the rays emitted of a hot body are of 

 several kinds, possessing different degrees of power to pass through 

 glass. 



He views the results, when the source of heat is' raised to the tem- 

 perature of luminosity, as forming one connected series with those 

 below that point, and thus conceives a gradual advance in the radiant 

 matter or agent from the state of simple heat towards that of light or 

 "luminous heat." 



6.) The theory adopted by De la Roche, as well as by Biot (Traite 

 de Phys., iv, G40) and Leslie, is that of one simple agent, which, as 

 the temperature of the source is raised, is gradually brought more 

 into the state of light, which on absorption is reconverted into heat. 

 At low temperatures it is wholly or nearly all stopped by transparent 

 eoreens. At increasing intensities more of it is enabled to pass in 

 the way of direct radiation. 



