396 



KNOBLAUCH ON RADIANT HEAT. 



and that reflected by black lac, under the same circumstances, a 

 deflection of 10°'64. That portion of the former which passed 

 through the plate of calcareous spar retained the needle of the 

 galvanometer at 14°*75 ; the portion of the latter permeating it, 

 at 20°*38, when the direct radiation of the reflected heat had 

 Table XXVIL 



Excepting black wood-charcoal and brown coal, none of the 

 surfaces of the varieties of coal used reflected sufficient heat to 

 allow of their being experimented upon by transmission. 



The diff"usion is most feeble in lamp-black and animal char- 

 coal. This renders the changes which the heat suffers on re- 

 flexion from the surfaces first mentioned the more remarkable. 

 Thus, the rays reflected by brown coal pass through red glass, 

 alum, calcareous spar and gypsum to a greater, those re- 

 flected by black vegetable charcoal through the same bodies to 

 a less extent than the unreflected rays of the lamp. The fol- 

 lowing numbers correspond to the quantities of heat which were 

 diffusely reflected upon the thermal pile at the same and 

 the most favourable position with regard to the Argand lamp, by 

 Indian ink, lamp-black, animal charcoal, fossil coal, coke and 

 graphite : — 



Table XXVIII. 



t Black Spanish leather e. g. produced under the same circumstances (by 

 the heat which it reflected) a deflection of 26° in the multiplier. 



