of the radiant heating Effects from tetrestrial Sources. 445 



III. (25.) I liave above adverted to all the sources of error 

 which occur to me as likely to have affected these results ; and 

 when these are taken into consideration, as well as the nature 

 of the experiments and apparatus, the accordance with the dif- 

 ferent results exhibited is perhaps as close as we can expect : 

 and it appears that all the different sets of experiments agree 

 in showing a very considerable difference in the ratio of the 

 effects produced on a smooth black and on an absorptive white 

 surface, by that part of the radiant effect transmitted through 

 glass, and by the total effect. If the total direct effect were 

 the result of one simple agent, the intervention of the glass 

 would, by intercepting some part of it, produce no other alte- 

 ration than a diminution of intensity; the ratio of the two 

 effects would remain unchanged. This distinction appears to 

 nie of some importance towards clearing our ideas respecting 

 the nature of the phsenomena, and thus affording an answer 

 to the question originally proposed in reference to some theo- 

 retical views, which, though boasting the sanction of high au- 

 thorit}'^, will be untenable if the validity of these i-esults be ad- 

 mitted. 



(26.) The general conclusions from all these experiments 

 may be thus recapitulated : 



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

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

 diation through glass, affects bodies in proportion to their 

 darhiess of colour^ without reference to the texture of their 

 surfaces. 



2nd. That which is intercepted produces a greater effect in 

 proportion to the absorptive nature of texture of the surface, 

 without respect to colour. These two characteristics are those 

 which distinguish simple radiant heat at all intensities. 



Thus then, when a body is heated at lower temperatures, it 

 gives off" only radiant heat stopped entirely by the most trans- 

 parent 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 

 radiant 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 tlie former, — a power which is capable of passing di- 

 rectly through transparent screens, and which acts more on a 

 smooth black surface tiian on an absorptive white one. 



(27.) This last sort of heat, whatever its nature may be, is 

 essentially different from simple radiant heat. It appears to 

 agree very closely with what the French ))hil()sophcrs term 

 calorique tianincux, and is, according to Prolessor Leslie's 



