388 



KNOBLAUCH ON RADIANT HEAT. 



gone by the reflexion of various bodies, I was obliged previously 

 to satisfy myself that even the altered position of the reflecting 

 sui'faces, as regards each other, did not produce a change in the 

 transmission of the heat by the diathermanous substances used 

 as tests. 



Experiment showed that e. g. on inserting the red glass, a 

 constant recess of 15'^ in the needle occurred, whether the direct 

 deflection of 24° was produced by the diifuse reflexion of the 



Table XXII. 



It is hence evident that the transmission of the diflfusely- 

 reflected heat by the diathermanous plates, within the limits of 

 these observations, is perfectly indrpendent of the distance, in- 

 clination and size of the reflecting surfaces, provided that before 

 the insertion the same action is exerted upon the instrument. 



The position of the diathermanous substances as regards the 

 thermal pile was the same as in the other experiments made 

 with them, in which it has already been shown (pp. 203 and 

 204) that free radiant heat was the real agent concerned. 



1. In the first series of experiments which I instituted to de- 

 termine the heat diffusely reflected by different bodies, I used 

 the Argand lamp which has been so frequently mentioned ; this 

 was kept at a constant level, had a double current of air and a 

 cylindrical wick, and was used without the glass chimney. To 

 allow of the reflecting bodies being accurately compared in dif- 

 ferent respects, they were divided into certain groups, which 

 were never so far extended as to cause any fear of altering the 

 conditions during the course of the experiments. 



I first examined a number of colouring matters, and very 



