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IX. The Bakerian Lectuke. — Contributions to Molecular Physics. — Being the Fifth 
Memoir of Researches on Radiant Heat. By John Tyndall, F.R.S. , Member of the 
Academies and Societies of Holland , of Geneva, Gottingen, Zurich, Halle, Marburg, 
Breslau, JJpsala, la Societe PhilomatMque , Paris, Cam. Phil. Soc. &c. ; Professor of 
Natural Philosophy in the Royal Institution. 
Received March 17, — Read March. 17, 1864. 
Contents. 
§ I. Preliminary considerations.— Description of apparatus. 
§ II. Absorption of radiant heat of a certain quality by eleven different liquids at five different thicknesses. 
§ III. Absorption of radiant heat of the same quality by the vapours of those liquids at a common tension. 
§ IT. Absorption of the same heat by the same vapours when the quantities of vapour are proportional to 
the quantities of liquid. — Comparative view of the action of liquids and their vapours on radiant heat. 
§ Y. Remarks on the chemical constitution of bodies with reference to their powers of absorption. 
§ YI. Transmission of radiant heat through bodies opaque to light. — Remarks on the physical cause of trans- 
parency and opacity. 
§ VII. Influence of temperature on the transmission of radiant heat. 
§ VIII. Changes of the position of diathermic bodies through changes of temperature of the same source. — 
Radiation from lampblack compared with that from platinum at the same temperature. 
§ IX. Radiation from gas-flames through vapours. — Reversals of position. 
§ X. Radiation from the flames of hydrogen and carbonic oxide through air and other media. — Influence of 
period with reference to absorption. 
§ XI. Radiation through liquids.— Influence of period. — Conversion of long periods into short ones. 
§ XII. Explanation of certain results of Melloni and M. Knoblauch. 
§ XIII. Radiation of hydrogen-flame through lampblack, iodine, and rock-salt. — Diathermancy of rock-salt 
examined. 
§ XIY. Connexion between radiation and conduction. 
p. 
The natural philosophy of the future must, I imagine, mainly consist in the investiga- 
tion of the relations which subsist between the ordinary matter of the universe and the 
ether, in which this matter is immersed. Regarding the motions of the ether itself, as 
illustrated by the phenomena of reflexion, refraction, interference, and diffraction, the 
optical investigations of the last half century have left nothing to be desired ; but as 
regards the atoms and molecules whence issue the undulations of light and heat, and 
their relations to the medium which they move, and by which they are set in motion, 
these investigations teach us nothing. To come closer to the origin of the ethereal 
waves — to get, if possible, some experimental hold of the oscillating atoms themselves 
— has been the main object of the researches in which I have been engaged for the 
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