348 Mr Ritchie 09i the Permeability of Screens 



Art. XXIX.-i— On the Permeability of Transparent Screens 

 of extreme tenuity by radiant Heat. * By William 

 Ritchie, A. M. 



The permeability of transparent screens by invisible radiant 

 heat flowing from a body of an elevated temperature was first 

 established by Professor Prevost of Geneva, and confirmed by 

 the elaborate experiments of M. Delaroche. As the results, 

 however, have been doubted by several philosophers, Mr 

 Ritchie considered that it would be desirable to place the 

 fact beyond the reach of controversy by new experiments. 



Exp. 1. A large glass globe having been blown so thin as 

 to be nearly iridescent, a small portion of it was fixed opposite 

 a circular hole about an inch in diameter, in a piece of tin 

 plate. A delicate air thermometer was then placed opposite 

 the disc of glass on one side of the plate, and a heated iron 

 ball opposite to the bulb on the other. The disc of glass was 

 kept uniformly beloiv the temperature of the surrounding air 

 by a current of cold air playing constantly against it. The 

 following results were then observed :— 



1. When the ball was at a low temperature, the thermome- 

 ter indicated no sensible effect. 



2. When the temperature of the ball was high, but not 

 visible in the dark, the effect on the thermometer was very 

 considerable, even when the ball was placed at a greater dis- 

 tance than formerly. 



" Here,'' says Mr Ritchie, " we have two sources of heat, 

 which, on account of the change of distance, would produce 

 equal efi*ects on the naked bulb of the thermometer ; but by 

 the intervention of a cold screen the effect of the former is al- 

 most annihilated, whereas the effect of the other is still very 

 considerable. This difference cannot possibly result from the 

 difference of temperature in the screen, which is kept as nearly 

 as possible at the same temperature by the influence of the 

 current of cold air. We are therefore unavoidably led to the 

 following conclusion,— ^Aa^ tJie progress of the heat was in the 



• This is a full abstract of Mr Ritchie's paper read before tlie Uoyal 

 Society on the 8th March 182(>. 



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