383 



Article XVII. 

 Note on the Reflection of Radiant Heat; by M. Melloni. 



(Read before the Academy of Sciences of Paris, November 2nd, 1835.) 



From tlie Jnnales de Chimie et de Physique, voi. lx. p. 402. 



Ai 



lLTHOUGH the researches of Leslie and Rumford established the 

 fact that the quantities of calorific rays reflected by different bodies 

 depend on the nature of those bodies and the polish of their surfaces, 

 the proportion of the reflected to the incident heat in each particular 

 case remained yet undetermined. The results presented by my experi- 

 ments on the immediate transmission of radiant heat enable us to solve 

 this question with considerable exactness. 



When the calorific rays fall perpendicularly on the anterior surface 

 of a diathermanous plate with parallel faces, they undergo a certain re- 

 flection, then penetrate into the interior of the plate, reach its further 

 surface, and, after there undergoing a second reflection, issue forth into 

 the air, pursuing their original direction. In certain cases there is no 

 internal absorption, and consequently the difference between the incident 

 heat and that transmitted, will in such cases be the value of the reflec- 

 tions produced at both surfaces. The substance in which this fact is 

 most distinctly perceived is rock salt. We know that plates of this sub- 

 stance, when very pure and well polished, transmit 0'923 of the incident 

 heat, whatever may be the thickness of the plates, the nature of the 

 rays, or the modifications which these may have previously suffered in 

 their passage through other plates. 



Let us suppose, for example, two plates of rock salt, the one measur- 

 ing one millimetre*, and the other ten millimetres f in thickness. Ac- 

 cording to what we have just stated, the transmissions of these two plates 

 will be equal; and if we imagine the thick plate divided into ten layers, 

 each one inillimelre thick, the absorbent power of the nine layers that 

 succeed the first will be of no appreciable value. Hence we may infer 

 that, if the rays suffer any absorption, it must be in the first layer. Let 

 us suppose for a moment that this takes place. On this hypothesis, the 

 molecules which constitute the first layer of one millimetre in thickness 

 will retain all the heat that is not completely transmissible by rock salt, 

 and the quantity of heat lost in the passage through either of the two 

 plates, that is, 1 — 0*923, or 0-077, will be but the sum of the rays ab- 

 sorbed or retained, and of those reflected at the two surfaces. Let the 



• 0-03937 in. t 0394 in. 



