1381.] 



Actio?i of Free Molecules on Radiant Heat. 



33 



fessor Daniel Oliver, F.L.S. ; Professor Henrv Enfield Roscoe, B.A., 

 LL.D. ; Warington W. Smyth, M.A. ; Henry Tibbats Stainton, 

 F.Gr.S.; Edward James Stone, M.A. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



The Bakerian Lecture — " Action of Free Molecules on Ra- 

 diant Heat, and its Conversion thereby into Sound,'" was 

 delivered by J. Tyndall, F.R.S. 



The following is an abstract : — 



The lectnre opens with a brief reference to the researches of Leslie, 

 Rumford, and Melloni. The labours of Tyndall and Magnus, as far 

 as thej bear upon the present subject, are then succinctly sketched, 

 their points of difference being signalised and briefly discussed. 

 This preliminary sketch is wound up by a reference to a recently 

 published paper by Lecher and Pernter, who, while supporting the 

 lecturer in the matter of gases, dissent from him in the matter of 

 vapours. These investigators are especially emphatic in affirming the 

 neutrality of aqueous vapour to radiant heat. Following Magnus, 

 they refer Tyndall' s results to what Magnus calls " vapour-hesion," 

 that is to say, to the condensation of the vapours on the surfaces of the 

 plates of rock-salt used to close the experimental tube, and on the 

 interior surface of the tube itself. 



In November, 1880, the lecturer's investigations in this field were 

 resumed. Former experiments were repeated and verified with divers 

 sources of heat, and with various experimental tubes — some polished 

 within, and others coated inside with lampblack. The results obtained 

 with the one class of tubes are substantially the same as those obtained 

 with the other. 



But even a coating of lampblack may be supposed to reflect a certain 

 amount of heat, hence the desirability of an arrangement whereby 

 internal reflection should be entirely abolished. This was accom- 

 plished in the following manner : — A spiral of platinum wire, ren- 

 dered incandescent by a voltaic current of measured strength, was 

 chosen as source of heat. An experimental tube 38 inches long and 

 6 inches in diameter had two circular apertures at its ends, closed 

 by transparent plates of rock-salt, 3 inches in diameter. The tube 

 was furnished with three cocks — one connected with a large Bianchi's 

 air-pump ; another with a purifying apparatus ; while through the 

 third vapours and gases could be admitted. Prior to entering the 

 tube, the calorific rays were sent through a very perfect rock-salt 

 lens, by means of which an image of the spiral was formed on the 



vol. xxxni. D 



