208 KNOBLAUCH ON RADIANT HEAT. 



black. I therefore preferred the above coating, so as not to in- 

 crease the delicacy of the apparatus at the expense of the accu- 

 racy of the comparison. 



For the sake of completeness, I examined the heating of the 

 substances mentioned, of unequal thickness by diflferent sources 

 of heat, applying for this purpose those which had always shown 

 the greatest difference, viz. an Argand lamp and a metallic 

 cylinder heated to about 212° F. 



Since besides lamp-black (p. 188) only metals (p. 206) absorb 

 all kinds of rays of heat to the same extent, the substances 

 mentioned for the present object could only be placed upon metal 

 if it was required to observe their action after having become 

 heated by different sources of heat, without the secondary in- 

 fluence of the sui'faces beneath, which in every other instance 

 would have been disproportionately heated. 



Experiment yielded the following result : — When by direct 

 radiation from an Argand lamp upon the thermal pile, a de- 

 flection of 60° was produced in the needle of the multiplier, if a 

 metallic plate was placed immediately before the thermoscope, 

 with its polished surface turned towards the source of heat, 

 whilst next the pile it was coated with paper, in a short time a 

 constant deviation of 10^'.5 was produced, which arose from the 

 metallic plate becoming heated. 



Thus the coated plate became more heated when the number 

 of layers of varnish covering it was increased. 



When the flame was exchanged for the cylinder at a dark red 

 heat, which produced the same direct deviation of 60°, and the 

 uncoated metallic screen was again inserted at the spot already 

 mentioned, the needle as before moved to 10°'5. However, it 

 was deflected to 17°' 5 when the metallic plate, covered with one 

 layer of varnish, was exposed to the rays from the heated cylin- 

 der, and to 20°' 75 when with eight layers of varnish. 



Thus in the latter case the heating was greater for each in- 

 dividual plate than with the first ; and increased fi'om that coated 

 with a single layer of varnish to that with eight layers in a 

 greater degree than in the experiment with the Argand lamp. 



The same phtenomenon occurred with black lac and white 

 lead. Thus, under exactly the same conditions, the heat com- 

 municated from a metallic sheath coated with a thin layer of lac, 

 when acted on by the rays of the Argand lamp, produced a 

 deflection of 14°'5 ; that from the same, covered with a thicker 



