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LVIII. Action of Free Molecules on Radiant Heat, and 

 its Conversion thereby into Sound. Bii John Tyndall, 

 F.R.S.* 



Contents. 



Page 

 Sect. 1. Introduction 435 



2. Partial Summary of previous Work 436 



3. Researches of Magnus , 446 



4. Experiments resumed : Verifications 459 



§ 1. Introduction, 



THE experimental researches of Rumford and Leslie raised 

 the subject of Radiant Heat to an extraordinary pitch 

 of interest and importance. Both of these philosophers 

 occupied themselves with what may be called superficial 

 emission and absorption. 



Melloni is to be regarded as the founder of our knowledge 

 of the transmission of radiant heat through solids and liquids. 

 Save in a passing inference, to be noticed immediately, 

 Melloni left untouched the gaseous form of matter — thinking, 

 probably, that gases and vapours, though their diathermancy 

 could hardly be supposed theoretically perfect, came in this 

 respect so near perfection as to be placed beyond the grasp of 

 laboratory experiment. It was doubtless the general pre- 

 valence of this conviction which caused this field of inquiry 

 to lie fallow for so many years after the discovery of the 

 thermo-electric pile. 



By an experimental arrangement characteristic of the 

 genius of the man, though, it may be, not quite equal to the 

 requirements of the problem, Melloni proved that the law of 

 inverse squares held good for radiant heat in air; and from 

 this he inferred the absence of all sensible absorption, by air, 

 within the distance embraced by his experiments f . Melloni 

 extended to radiation his conclusion regarding absorption. 

 "On ne connait," he writes, " aucun fait qui demontre 

 directement le pouvoir emissif des fluides elastiques purs et 

 transparents " t« Such was Melloni's relation to the subject 

 now before us. 



* Communicated by the Author, having been read before the Royal 

 Society, January 23, 1882. The substance of this paper was delivered 

 orally as the Bakerian Lecture on November 24, 1881. 



t "Pour un intervalle de cinq a six metres, l'air n'exerce aucune 

 absorption sensible pour le rayonnement des corps chauds." — La Ther- 

 mochrose, p. 136. 



% Annates de Chimie et de Physique, vol. xxii. p. 494, 



