1861.] Professor Tyndall on Radiant Heat. 295 



M6moires dee Sciences Politiques, &c. Tome IX. 4to. 1859. 

 Memoires : 7e Serie. Tome II. Nos. 4-7. Tome III. No. I. 4to. 1860. 

 Bulletins: Tome II. Nos. 1-3. 4to. 1860. 

 Vereins zur Befdrdervng des Gewerbfleisaea in Prci«sCTi— Verhandlungen, Juli und 

 Aug. 1860. 4to. 



1861. 



WEEKLY EVENING MEETING, 



Friday, January 18, 1861. 



William Robert Grove, Esq. Q.C. F.R.S. Vice-President, 

 in the Chair. 



John Tyndall, Esq. F.R.S. 



PBOFESSOB OF NATUBAL PHILOSOPHY, KOTAL INSTITtTTION. 



On the Action of Gases and Vapours on Radiant Heat. 



The discourse commenced by a reference to the researches of Leslie, 

 Forbes, and Knoblauch ; but more especially to the admirable inves- 

 tigations of Melloni on Radiant Heat. These eminent men had left 

 the gaseous form of matter practically untouched, and to extend our 

 knowledge into this wide region was the object of the investigation 

 on which the present discourse was founded. 



The apparatus made use of, and which was applied in the experi- 

 ments of the evening, consists of the following parts : — 



1. A copper cube C, containing water kept constantly boiling, 

 and one of whose faces, coated with lamp-black, forms the source of 

 radiant heat. 



2. A brass tube, 2 • 4 inches in diameter, which is divided into two 

 portions, a and /3. 



a. The portion of the tube intended to receive the gases and vapours ; 

 it is stopped air-tight at its two ends by plates of rock-salt, and is 

 attached to a good air-pump, by which it can be exhausted at pleasure. 

 The length is 4 feet. 



/8. Ai? air-tight chamber between the tube a and the cube C. It 

 is kept constantly exhausted, and the calorific rays therefore pass from 

 the radiating plate through a vacuum into the tube, thus retaining the 

 quality which belonged to them at the moment of emission. 



x2 



