236 KNOBLAUCH ON RADIANT HEAT. 



the same proceeding as the adiathermanous surface. It is placed, 

 as regards the Argand lamp and the thermal pile, in such a 

 manner, that, as befoi-e, a deflection of the needle to 35° is pro- 

 duced, which in this case may possibly ai'ise from heat of the 

 flame which passes through it, as well as from the ivory plate 

 becoming heated. The question then is, if the rays of heat, 

 which under such circumstances have deflected the needle to 

 35", will permeate the diathermanous substance in the same pro- 

 portion as that previously emitted by the adiathermanous sur- 

 face, i. e. whether now also, <?. g. on inserting the red glass, a 

 recession of the needle to 10°'25 is obtained. If this is the case, 

 and occurs in the same way in all other diathermanous media 

 used for testing, e. g. also with blue glass, alum, rock salt, cal- 

 careous spar and gypsum, the ivory plate is adiathermanous ; 

 for then only, as we know (p. 225 and 226), do we find no dif- 

 ference in this respect. But if, the second time the rays of heat 

 do not pass in the same manner as before through the diather- 

 manous bodies, we obtain in the case of a single one only a dif- 

 ferent deflection after the insertion as before, it is a proof of the 

 diathermancy of the plate of ivory. Experiment gave 13°"c2 on 

 inserting the red glass, and similar differences with other dia- 

 thermanous substances (Table XVII.). Ivory is thus diatherma- 

 nous. The criterion for deciding the question is therefore briefly 

 this : — 



" If the heat which impinges upon the plate under examina- 

 tion, when exposed to an Argand lamp, cannot by transmission 

 be distinguished from the heat of any other known adiatherma- 

 nous body, the plate itself is adiathermanous. If differences do 

 occur, it is diathermanous." 



This position could not be instituted until it was known that 

 the peculiar heat of different substances did not produce the same 

 differences. 



The temperature of the bodies compared of coarse should not 

 be allowed to exceed 234° F. 



That it is not a matter of indifference whether in this investi- 

 gation an Argand lamp or any other source of heat is used, 

 may be seen from the experiments detailed in Table XVIII., 

 in which adiathermanous and diathermanous bodies could not 

 be distinguished from one another, on exchanging the Argand 

 lamp for a metallic cylinder at 212" F., the general method of 

 proceeding being the same. 



