650 THE POPULAR SCIENCE MONTHLY. 



SOUND AND RADIANT HEAT * 



By Professob JOHN TYNDALL. 



THE Bakerian lecture of 1881 before the Royal Society opens with 

 a brief reference to the researches of Leslie, Rumford, and 

 Melloni. The labors of Tyndall and Magnus, as far as they bear upon 

 the present subject, are then succinctly sketched, their points of dif- 

 ference being signalized 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 vapors. These investiga- 

 tors are especially emphatic in affirming the neutrality of aqueous 

 vapor to radiant heat. Following Magnus, they refer Tyndall's re- 

 sults to vapor-hesion, that is to say, to the condensation of the vapors 

 on the surfaces of the plates of rock-salt used to close the experi- 

 mental 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 experimental tubes, some polished within, 

 and others coated inside with lamp-black. The results obtained with 

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

 with the other. 



But even a coating of lamp-black may be supposed to reflect a cer- 

 tain 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, rendered 

 incandescent by a voltaic current of measured strength, was chosen 

 as source of heat. An experimental tube thirty-eight inches long and 

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

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

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

 air-pump, another with a purifying apparatus, while through the third 

 vapors 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 most distant plate 

 of rock-salt. To obtain the image with clearness, the spiral was first 

 rendered highly luminous, and afterward reduced, by the introduction 

 of resistance, to the required temperature. In this way a calorific 

 beam was sent along the axis of the experimental tube without at all 

 impinging upon its interior surface. No reflection came into play ; 

 no absorption by hypothetical liquid films, coating the internal surface, 



* Abstract of the Bakerian Lecture of 1881, delivered by Dr. Tyndall, F. R. S., on 

 the " Action of Free Molecules on Radiant Heat, and its Conversion thereby into Sound." 



