204 



KNOBLAUCH ON RADIANT HEAT. 



4. When the substances are inserted in their oi'dinary state 

 before the thermal pile, the needle recedes to a point which 

 it does not leave during the observation ; whilst whenever they 

 are coated with lamp-black, and thus become more heated by 

 absorption, an increase of this deflection is perceived. 



Moreover, my results agree perfectly, as far as they are com- 

 parable, with those of Melloni, who during his observations has 

 convinced himself partly in the same way, that the heat emitted 

 by the screens bears no proportion to that transmitted by them. 

 A diffuse transmission cannot have occurred in the cases men- 

 tioned, as the diathermanous bodies were all polished as highly 

 as possible. 



Their thickness was a matter of no iinportance, since they 

 were not to be compared with each other, but merely served for 

 the investigation as regards the sources of heat. 



That the different form and size of the sources of heat which 

 have been compared did not induce any differences in the trans- 

 mission, has been proved by direct experiments ; for it was 

 found that when the needle of the multiplier was deflected to 

 50° by direct radiation upon the pile, on introducing the red 

 glass it receded to 21°'25 — 21°, when either a Leslie's cube the 

 sides of which were 4 centim., 8 centim,, or 15 centim. 7 millim., 

 or a cylinder 17 centim. high and 3 centim. in diameter was 

 used as a source of heat at 212° F. The direct deflection being 

 50°, the same result of a return of the needle to 31°*5 was ob- 

 tained, whether a small glass spirit-lamp or a large Berzelius's 

 lamp was used; and the same deviation of 37°"5, both for the 

 small flame of a wax-light and for the large one of an Argand 

 lamp. 



The following summary w hich contains the arithmetical means 

 of every three observations, shows that this was also the case 

 with other diathermanous bodies : — 



Table V. 



