KNOBLAUCH ON UADIANT HEAT. 227 



Tn this case, as before (p. 196 to 202 and 224), it was a matter of 

 indifference whether the radiating substances became heated in 

 a higher or less degree ; for the portion of heat which passed 

 through the diathermanous substances remained the same whether 

 the direct deflection of 35° was produced by placing the disc to 

 be heated nearer the source of heat and further from the ther- 

 moscope, or nearer the pile and at a greater distance from the 

 source of heat. 



It is thus evident from these observations, that the heat radiated 

 by different bodies always passes through the diathermanous media 

 used in their investigation in the same proportion, be the rays of 

 heat, by the absorption of which they are heated, ever so different. 



When in these researches the object was to investigate 

 the transmission of the heat emitted from certain bodies by 

 the substances used for examining them, diathermanous bodies 

 of course could not be used as the heating agents in the method 

 described ; for when they were placed, as in the previous ar- 

 rangement of the apparatus, between the original sources of heat, 

 e. g. the Argand lamp and the thermoscope, not only the heat 

 from the flame radiated by them, but also that passing through 

 them reached the pile. 



Experiments of this kind could not therefore aid in deciding 

 the question especially under consideration ; but they were 

 adapted to test the accuracy of the method itself, used for ascer- 

 taining the identity or dissimilarity of certain rays of heat. 



Thus in the manner pointed out, a number of rays were ob- 

 tained which differed in their capability of passing through dia- 

 thermanous bodies *, and the proportion of which to each other 

 was dependent upon the nature of the body in which they ap- 

 peared. Such a mixture of different kinds of heat, according as 

 it belonged to the one or the other substance, must therefore per- 

 meate the media under investigation to an unequal extent. 



The next question was, whether these differences in the trans- 

 mission, which were pre-supposed by theory to exist, would also 

 be perceptible in red and blue glass, alum, rock salt, calcareous 

 spar and sulphate of lime, in cases in which they were only ad- 

 mitted to be small. 



The experiment simply consisted in placing a slightly diather- 

 manous body, e.g. a plate of ivory 1*7 millim. in thickness, on 

 that side of a perforated screen next which the Argand lamp 



• Compare in Table XV., p. 225 and 220, the deflections for the heated 

 bodies after the insertion with those for the Argand lamp. 



