Philosophical Transactions, 305 



have adopted at the sup^gestion of the Rev. W. Conybeare) will 

 not, it is presumed, be deemed objectionable." 



ix. An Experimental Inquiry into the Nature of the radiant 



heating Effects from terrestrial Sources. By Baden Powell, M.A., 



F.R.S., of' Oriel College, Oxford. 

 The general conclusions to be deduced from the experimental 

 details of this paper are as follow : — 



1st. That part of the heating effect of a luminous hot body, 

 which is capable of being transmitted in the way of direct radia- 

 tion through glass, affects bodies in proportions to their darkness of 

 colour, without reference to the texture of their surfaces. 



:id. That which is intercepted produces a greater effect in pro- 

 portion to the absorptive nature of texture of the surface,"" without 

 respect to colour. These two characteristics are those which dis- 

 tinguish simple radiant heat at all intensities. 



Thus when a body is heated at lower temperatures, it gives off 

 only radiant heat stopped entirely. by the most transparent glass, 

 and acting more on an absorptive white surface than on a smooth 

 black one. 



At higher temperatures the body still continues to give out ra- 

 diant heat, possessing exactly the same characters. 



But at a certain point it begins to give out light : precisely at 

 this point it begins also to exercise another heating power distinct 

 from the former ; a power which is capable of passing directly 

 through transparent screens, and which acts more on a smooth 

 black surface than on an absorptive white one. 



This last sort of heat, whatever its nature may be, is essen- 

 tially different from simple radiant heat. It appears to agree 

 very closely with what the French philosophers term " calorique 

 lumineux," and is, according to Professor Leslie's theory, a con- 

 version of light into heat. These views of the subject are cer- 

 tainly gratuitous assumptions. We have no right whatever to 

 identify those two agents, or to suppose that, because a heating 

 effect very closely accompanies the course of the rays of light, 

 the light is therefore converted into heat ; but the theories 

 above alluded to, seem to regard the whole heating effect of a 

 luminous body aa of this latter character. In this particular, the 

 present inquiry has led us to an essential distinction ; and if the 

 experiments are to be relied upon, this peculiar sort of heat con- 

 stitutes only a part of the total effect. These results do not in- 

 deed present so simple a theory as that alluded to, but they apply 

 Very obviously to the explanation of many phenomena recorded 

 by v{y»^s experimenters. 



The peculiar heat above spoken of, and which, for the sake of 

 distinction and brevity, we may call " transmissible heat," is si- 

 milar to that which acts in the solar rays, and which there con- 



VoL. XIX. X 



