210 KNOBLAUCH ON RADIANT HEAT. 



hindrance. Each of these layers then communicates its ca- 

 loric by conduction and radiation to the neighbouring ones. 

 The amount of heat imparted in this manner to any body there- 

 fore increases in proportion as the number of the absorbing 

 layers increases ; but it attains its maximum as soon as these 

 acquire a thickness beyond which the heat cannot penetrate 

 either by radiation or conduction. 



In the series of experiments which has been detailed, the 

 thickness was never so great but that every coat laid on became 

 heated, and that the heat of all could act upon the metallic sur- 

 face, which emitted the rays against the thermal pile by means 

 of the paper covering ; but in the experiments of Leslie and 

 Melloni, the screens inserted (which were only diathermanous 

 in the thinnest layers) were so thick, that a small portion only 

 of the heat from their anterior surfaces reached the side next the 

 thermoscope ; and hence its action upon the latter must have 

 been diminished to the same extent as this portion was weakened 

 by increasing the thickness. 



The limit at which the heating of a body ceases to increase in 

 proportion to the diminished thickness, is determined for one 

 and the same source of heat by the substance, and for one and 

 the same substance by the nature of the source of heat. We 

 have reserved the more ininute examination of this point in cer- 

 tain cases for a future investigation. 



Melloni considered that it is impossible to detect the elevation 

 of temperatui*e which thin diathermanous plates experience from 

 radiant heat, and therefore concluded indirectly regarding their 

 becoming heated. In the experiments detailed, I succeeded in 

 proving it by the direct method, and in experimentally confirming 

 Melloni's conjectures in a palpable manner, that the temperature 

 of a body, when the thickness increases, is more raised the less it 

 is diathermanous to the 7'ays tra^ismitted to it. Thus, as has 

 been already mentioned, the observations contained in the table 

 at p. 209 always show, in the case of the same body, with the 

 cylinder at a dark red heat, a greater increase in the heat re- 

 quired in proportion to the thickness than in the Argand lamp ; 

 whilst direct ti'ansmission has shown that the heat of the former 

 is transmitted in a less degree than that of the latter by white 

 varnish and black lac. 



That diathermanous bodies, as has hitherto been only sup- 

 posed, in reality become most heated by those rays which pene- 



