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VI. Action of Free Molecules on Radiant Heat, and its Conversion thereby into Sound. 
By John Tyndall, F.R.S. 
Received January 23, 1882. # 
Contents. 
PAGE. 
§. 1. Introduction ... 291 
§. 2. Partial Summary of previous work . 292 
§. 3. Researches of Magnus . ..' 301 
§. 4. Experiments resumed : Verifications. 311 
§. 5. New Experiments. Hypothesis of Internal Eilms. 314 
a. The Experimental Tube . 314 
b. The Plates of Rocksalt. 319 
§. 6. Conservation of Molecular Action. 323 
§. 7. Thermal Continuity of Liquids and Vapours . 327 
§. 8. Rhythmic Absorption of Radiant Heat by Gases and Vapours. 332 
§. 9. Manometric measurements. 338 
§.10. Application of Results to Meteorology. 343 
§. 1. Introduction. 
The experimental researches of Btjmeord and Leslie raised the subject of Itadiant 
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 theo¬ 
retically perfect, came in this respect so near perfection as to be placed beyond the 
grasp of laboratory experiment. It was doubtless the general prevalence of this con¬ 
viction 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 ali sensible absorption, by air, within the distance embraced by his experk 
* The substance of this paper was delivered orally as the Bakerian Lecture on November 24, 1881. 
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