262 DISCUSSIONS IN CLIMATOLOGY. 



till we reached a film so thin as to embrace but only- 

 one particle in its thickness, its rate of radiation would 

 be proportionate to its temperature, or, in other words, 

 it would obey Newton's law. Prof. Balfour Stewart's 

 explanation is this : As all substances are more cliather- 

 manous for heat of high than of low temperatures, 

 when a body is at a low temperature only the exterior 

 particles supply the radiation, the heat from the interior 

 particles being all stopped by the exterior ones, while 

 at a high temperature part of the heat from the interior 

 is allowed to pass, thereby swelling the total radiation. 

 But as the plate becomes thinner and thinner, the 

 obstructions to interior radiation become less and less, 

 and as these obstructions are greater for radiation at 

 low than high temperatures, it necessarily follows 

 that, by reducing the thickness of the plate, we assist 

 radiation at low more than at high temperatures. 



If this be the true explanation why the radiation of 

 bodies deviates from Newton's law, it should follow 

 that in the case of gases where the particles stand at 

 a considerable distance from one another, the obstruc- 

 tion to interior radiation must be far less than in a 

 solid, and consequently that the rate at which a gas 

 radiates its heat as its temperature rises, must increase 

 more slowly than that of a solid substance. In other 

 words, in the case of a gas, the rate of radiation must 

 correspond more nearly to the absolute temperature 

 than in that of a solid ; and the less the density and 

 volume of a gas, the more nearly will its rate of radia- 

 tion agree with Newton's law. The obstruction to 

 interior radiation into space must diminish as we 

 ascend in the atmosphere, at the outer limits of which, 

 where there is no obstruction, the rate of radiation 

 should be pretty nearly proportional to the absolute 

 temperature. May not this to a certain extent be the 



