1 835.] the Advancement of Science, 209 



be interpreted with reference to this distinction; and 

 possibly the consideration of it may remove some of the 

 apparent anomalies. 



And further, if the distinction which he established in the 

 case of luminous hot bodies do really continue to exist, (as it 

 seems it must do unless there be a breach of the law of con- 

 tinuity) at temperatures below that of visible luminosity, 

 (itself a very undefined point) then it ought to be rendered 

 manifest by a careful repetition of the same fundamental 

 experiment, with very delicate thermoscopes, (either Mel- 

 loni's or such as that used by Dr. Hudson) by which a 

 smaller, though probably very minute difference of ratio, 

 would be found in the effects on a black and white thermo- 

 scope, with and without a glass screen. 



Should this be found to be the case, in all cases of reflec- 

 tion, polarization, &c., with non-luminous sources, it will 

 henceforth become necessary to distinguish to which of the 

 two heating causes they belong ; every analogy would lead 

 us to suppose they belong essentially to that of the two 

 heating agents which is so closely associated with the light, 

 that it seems rather a property of the light than any sepa- 

 rate agent. The length of the undulations which Professor 

 Forbes has shewn may by analogy be calculated, would be- 

 long to this species of radiant heat, and be simply such 

 undulations in the setherial medium as are too large to 

 affect our visual organs ; whilst the other portion may 

 very probably be as Sir J. Leslie conjectured, merely a 

 conveyance of heat by the air ; this would agree with what 

 is observed of its rapidly diminishing as the distance in- 

 creases, and being incapable of permeating screens except 

 by conduction : this, perhaps, may account for the irregu- 

 larities found by Sir J. Leslie in the position of the forces 

 of a reflector for heat. Whereas the other sort is conveyed 

 unimpaired to the same distances as the light, and only ex- 

 cited when it is absorbed. A vacuum absolutely perfect, de- 

 monstrably, can never be formed with the air pump, and the 

 torrecellian, even if free from air and the vapour of mercury 

 is not large enough for satisfactory experiments, but even 

 if it should be contended that the radiation through vacuum 

 has been proved, we must now say to which sort of he^t it 

 applies. By this distinction the theories of Leslie, De la 



VOL. II p 



