KNOBLAUCH O.V RADIANT HEAT. 217 



looked to what extent this is strictly admissible. It holds gooJ 

 unconditionally in the case of one and the same body ; i. e. all 

 agents which increase or diminish its radiation, also increase 

 or diminish its absorption, and vice versa. Thus scratching- the 

 surface of a substance increases both its radiating and absorbing 

 power when it exposes softer parts of it, diminishes both when 

 it condenses the same parts, and has no influence upon it 

 when the hardness of the body is left unchanged. As we have 

 seen, also, the radiating and absorbing power is increased to 

 a certain extent by increasing the thickness of bodies, but is 

 diminished by diminishing it. 



The comparison of the two phaenomena does not however 

 apply generally to different substances, i. e. a body which, at a 

 definite heating power, for instance, exhibits a higher radiating 

 power than another, does not therefore possess a better absorp- 

 tive power ; for the proportion of the amounts of heat absorbed 

 by it changes with the nature of the rays of heat which reach 

 it ; and also the quantities of heat emitted by them appear under 

 different conditions to alter in a different manner. 



Melloni maintains that a substance which at a given tempera- 

 ture (212° F.) emits more heat than another, always in the same 

 proportion absorbs more than it when exposed to the same tem- 

 perature (212° F.). However, it is a question whether this con- 

 clusion is satisfactorily proved by the experiments of this distin- 

 guished philosopher, because they merely refer to six bodies, two 

 of which moreover (lamp-black and metal) absorb equally all 

 kinds of calorific rays, and hence cannot enter into consideration 

 in a question of the unequal absorption of heat emitted from 

 different sources. 



The observations of Rumford and Leshe on this point are not 

 conclusive, because they do not allow of an accurate comparison 

 of different bodies in regard to the radiation and absorption of 

 heat ; nor do those of Ritchie, which appear to prove the absolute 

 similarity of the two phaenomena, but in fact, as far as they are 

 published, merely extend to lamp-black and metal; and we have 

 already explained why in this case they cannot lead to a general 

 conclusion. 



3. Those experiments which have hitherto been made do not 

 give any explanation of a question the solution of which does 

 not appear to me void of interest ; it is this : — 



Does the radiating power of one and the same body vary ac- 



a 2 



