KNOBLAUCH ON RADIANT HEAT. 385 



the chief results of which M. Magnus did me the honour of 

 laying before the meeting of the Royal Academy of Berlin on 

 the 29th of May 1845, a notice of which was taken from the 

 Monthly Report of the Berlin Academy, and inserted in Pog- 

 gendorff's Annalen, vol. Ixv. p. 581-592. In the following me- 

 moir I shall give the details of this investigation. 



It has been already mentioned, that of the two means which 

 we possess of detecting differences in heat, that of radiation 

 through diathermanous bodies is preferable to absorption, and 

 at p. 227-230 instances are given of the great delicacy of this 

 test-method. 



I therefore adopted it in the present instance also, and exa- 

 mined "whether radiant heat permeates the same diatherma- 

 nous media in dissimilar proportions, according as it is unre- 

 flected or diffusely reflected by different bodies." 



Great differences were in fact found. Thus, when the heat 

 of an Argand lamp radiated upon the pile, so that the needlff of 

 the multiplier was deflected 25°, it receded to 15°* 19 when a 

 plate of calcareous spar 3' 7 millim. in thickness was inserted 

 between the source of heat and the thermoscope. The 15°"19, 

 as is known, arose from the heat transmitted by the calcareous 

 spar. But when the heat of the lamp, diffusely reflected by a 

 carmine-surface, had produced the deflection of 25°, the needle 

 receded to 22°'31, when the same plate of calcareous spar was 

 inserted at exactly the same spot between the reflecting surface 

 and the thermal pile. The heat reflected by the carmine was 

 therefore transmitted by the calcareous spar comparatively 

 better than that unreflected. The same was the case with other 

 diathermanous media. 



The rays of heat diffusely reflected by a large number of bodies 

 were compared, in the manner above described, both with that 

 unreflected and with each other (as regards their transmission 

 through diathermanous substances). However, before proceed- 

 ing to the results of these experiments, I must premise some 

 remarks upon the method of proceeding which was adopted. 



To ensure the action of reflected heat alone upon the thermo- 

 scope, care must of course be taken to avoid heating the reflecting 

 bodies. This was effected by employing them in the form of 

 the lateral surfaces of metallic cubes which contained water of 

 the temperature of surrounding bodies. Thus, those which were 

 to be compared with each other were spread upon different cubes, 



VOL. V. PART XJX. 2 D 



