KNOBLAUCH OX RADIANT HEAT. 



209 



layer, 18^*12 ; and by the rays of the cylinder at a dark red heat, 

 the first a deviation of 18°-62, the latter of 22°-12. It must here 

 be taken into consideration, that the power which deflects the 

 galvanometer- needle a certain number of degrees higher i?, greater 

 than that which causes it to deviate the same number of degrees 

 lower. 



The numbers found with the coating of white lead are given in 

 the subjoined table, which also contains values for other thick- 

 nesses of the bodies spoken of, and a different direct deflection 

 from that mentioned (in each case the arithmetic mean of two 

 obsenations) : — 



Table VII. 



6-50 



6-50 



10-50 



10-50 I 



Argand lamp. Deflection from direct radiation 35°. 

 8-25 j 8-25 I 8-25 | 8-25 | ;-12 | 8-25 | 8-62 | 9-50 | 



Metallic cylinder at a dark red heat. 

 900 I 9-25 I 9-50 ] 9-50 | 9-12 [ 987 | 11-62 ] 1200 | 



7-25 

 8-75 



;oo 



9-62 



Argand lamp. Deflection from direct radiation 60°. 

 14-50 I 15-12 I 15-62 | 15 75 | 14-50 | 1625 | 17-37 I 18-12 | 1612 | 1850 



Metallic cylinder at a dark red heat. 

 17-50 I 18-12 I 20-12 | 20-75 | 18-62 | 20-25 | 21-37 | 2212 | 17-00 | 19-50 



The differences which are thus rendered evident could only 

 arise from the heat which is absorbed by the substances applied 

 as coatings, and in this manner communicated to the metallic 

 plates. It is thus shown, that the substances employed become 

 heated, within the limits of this experiment, to a degree the 

 extent of which is jjroportionate to their thickness. 



This observation is directly opposed to the experiments made 

 by LesHe and Melloni upon other substances. Nevertheless, 

 both are correct. The cause of this difference is, that in ray 

 experiments I have not yet attained the limit beyond which 

 these earlier experimenters had already passed. For the con- 

 nexion of the pha^nomena is as follows : — If we expose a body 

 to rays emanating from a source of heat, those which are not 

 reflected from its surface penetrate it, and impart heat to one 

 layer after another, as long as they pass through it without 



