Mr Ritchie on Radiant Heat. l37 



was observed by the celebrated Lambert, though both he and 

 Cullen seem to me to have given a false explanation of the 

 cause ; Pyrometria, sect. 49^. Saussure notices the same fact, 

 and remarks: " Le grand geometre Lambert rapport cette 

 meme experience dans sa pyrometrie ; et il ajoute, que Texpe- 

 rience reussit plus silrement et donne un difference de chaleur 

 plus grande lorsqu''on pompe Pair avec diligence, et que le 

 recipient n'est pas trop petite."" Essais sur V Hygrometrie, 

 p. 232. Now this fact, which by no means receives a satis- 

 factory explanation from the theory of aerial pulsations, is 

 easily accounted for by the idio-repulsive theory. If the 

 exterior film of caloric be pressed towards the body by the 

 ambient air, this pressure will evidently act as an antago- 

 nist force to the repulsive energy of the molecules of heat. 

 If this process be diminished or removed, the atoms of heat 

 will act on each other with a less restrained force, and a more 

 copious radiation will be the necessary consequence. If this 

 pressure be augmented by condensing the air there will not 

 only be a greater number of aerial molecules, but they will 

 also press with a greater force against the exterior film of 

 heat. The atoms of caloric will therefore be much restrained 

 in their mutual actions, and consequently the current of ra- 

 diant heat will be much diminished. Were the air to be 

 highly condensed, and the temperature of the body proportion- 

 ably low, the two forces might become equal, and radiation 

 completely prevented, as is the case -wdth a heated body not 

 visible in the dark when plunged into water. Dr Wells, in 

 his elegant Essay on the Formation of Dew ^ has given a similar 

 explanation of the process by which water is converted into 

 ice, on the extensive plains of Hindostan, when the tempera- 

 ture of the air is many degrees above the freezing point. 

 Evaporation he proves is insufficient to produce the effect. 

 Radiation is also necessary to reduce the temperature of the 

 water to the freezing point.* 



In order to account for the striking difference between the 

 radiating powers of different surfaces, Professor Leslie assumes 

 that " air never comes into actual contact with any surface, but 



* Esmy on Dew% from p. 261 to p. 279. 



