92 Mr. J. Croll on Ocean-currents in relation to 



were made on solids ; but, from certain results arrived at by Dr. 

 Balfour Stewart, it would seem that the radiation of a material 

 particle may be proportionate to its absolute, temperature. This 

 physicist found that the radiation of a thick plate of glass in- 

 creases more rapidly than that of a thin plate as the temperature 

 rises, and that, if we go on continually diminishing the thickness 

 of the plate whose radiation at different temperatures we are 

 ascertaining, we find that as it grows thinner and thinner the 

 rate at which it radiates off its heat as its temperature rises be- 

 comes less and less. In other words, as the plate grows thinner 

 and thinner its rate of radiation becomes more and more pro- 

 portionate to its absolute temperature. And we cau hardly re- 

 sist the conviction that if we could possibly go on diminishing 

 the thickness of the plate till we reached a film so thin as to 

 embrace but only one particle in its thickness, its rate of radia- 

 tion would be proportionate to its temperature. Dr. Balfour 

 Stewart has very ingeniously suggested the probable reason why 

 the rate of radiation of thick plates increases with rise of tem- 

 perature more rapidly than that of thin. It is this : all sub- 

 stances are more diathermanous for heat of high temperatures 

 than for heat of low temperatures. "When a body is at a low 

 temperature, we may suppose that only the exterior rows of par- 

 ticles supply the radiation, the heat from the interior particles 

 being all stopped by the exterior ones, the substance being very 

 opaque for heat of low temperature ; while at a high tempera- 

 ture we may imagine that part of the heat from the interior 

 particles 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 obstruc- 

 tions are greater for radiation at low temperatures than for ra- 

 diation at high temperatures, it necessarily follows that, by re- 

 ducing the thickness of the plate, we assist radiation at low tem- 

 peratures more than we do at high. 



In a gas, where each particle may be assumed to radiate by 

 itself, and where the particles stand at a considerable distance 

 from one another, the obstruction to interior radiation must be 

 far less than in a solid. In this case the rate at which a gas 

 radiates off its heat as its temperature rises must increase more 

 slowly than that of a solid substance. In other words, its rate 

 of radiation must correspond more nearly to its absolute tem- 

 perature than that of a solid. If this be the case, a reduction in 

 the amount of heat received from the sun, owing to an increase 

 of his distance, should tend to produce a greater lowering 

 effect on the temperature of the air than it does on the tempera- 

 ture of the solid ground. But as the temperature of our cli- 

 mate is determined by the temperature of the air, it must fol- 



