KNOBLALCH ON RADIANT HEAT. 207 



detailed, that this heating, the intensity of the heat received by 

 radiation being the same, is perfectly independent of the tem- 

 perature of its source, but is occasioned by the nature of those 

 absorbing substances, which are more susceptible of some rays 

 than of others. 



The influence of the thickness of the bodies exposed to the 

 rays of heat upon their becoming heated has scarcely hitherto 

 been investigated. Leslie remarked that metals of different 

 thickness became heated to the same extent ; but that wooden 

 screens, which he placed before a heated cube, were less heated 

 in proportion to their thickness. Thus his thermoscope indi- 

 cated — 



20° behind a plate of pine-wood ^th inch in thickness, 

 15° ... ... ^th 



9° ... ... 1 



Melloni also found, by means of the thermo-multiplier, that 

 thick paper became less heated than thin. 



However, these experiments did not appear to me to decide 

 the question — 



In lohat relation the heating of a body stands to its thickness ; 

 which has therefore formed the object of the following investi- 

 gation. 



Whilst in making the observations in the previous section I 

 endeavoured to ascertain the influence exerted by the media 

 placed between the source of heat and the thermal pile when 

 heated, upon the latter ; in this case I took special care to make 

 it as conspicuous as possible, and so that it acted exclusively upon 

 the thermoscope. I therefore placed the bodies to be heated 

 immediately before the latter, and furnished them on that side 

 next the pile with a coating impervious to direct rays. 



The substances which I used in these experiments were colour- 

 less transparent varnish, black, opake, but diathermanous lac, 

 and white lead, which is usually regarded as adiathermanous. 

 I placed these in layers of different thickness upon thin metallic 

 discs in every respect alike. To improve the dispersion from 

 the latter after it has become heated, I coated them on the sides 

 next the pile with paper. Lamp-black would certainly have 

 been more effective for this purpose ; but it is scarcely possible 

 to lay it upon several plates in exactly the same manner, which 

 would have been indispensable, because, as Melloni has shown, 

 the dispersion varies with the thickness of the layers of lamp- 



