332 Prof. Tyndall on Luminous and Obscure Radiation. 



the solid carbon of the luminous flame. The same is true for 

 the flame of a Bunsen's burner; the moment the air (which 

 destroys the solid carbon-particles) mingles with the gas-flame, 

 the radiation falls considerably. Conversely, a gush of radiant 

 heat accompanies the shutting out of the air which deprives the 

 gas-flame of its luminosity. When, therefore, we introduce a 

 platinum wire into a hydrogen-flame, or carbon-particles into a 

 Bunsen's flame, we obtain not only waves of a new period, but 

 also convert a large portion of the heat of convection into the 

 heat of radiation. 



11. The action was still very sensible when the distance of the 

 pile from the red end of the spectrum on the one side was as 

 great as that of the violet rays on the other, the heat- spectrum 

 thus proving itself to be at least as long as the light-spectrum. 



12. Bunsen and Kirchhoff have proved that, for incandescent 

 metallic vapours, the period of vibration is, within wide limits, 

 independent of temperature. My own experiments with flames 

 of hydrogen and carbonic oxide as sources, and with cold aqueous 

 vapour and cold carbonic acid as absorbing media, point to the 

 same conclusion*. But in solid metals augmented temperature 

 introduces waves of shorter periods into the radiation. It may 

 be asked, " What becomes of the long obscure periods when we 

 heighten the temperature ? Are they broken up or changed into 

 shorter ones, or do they maintain themselves side by side with 

 the new vibrations ? " The question is worth an experimental 

 answer. 



13. A spiral of platinum wire suitably supported was placed 

 within the camera of the electric lamp at the place usually occu- 

 pied by the carbon points. This spiral was connected with a 

 voltaic battery ; and by varying the resistance to the current, it 

 was possible to raise the spiral gradually from a state of darkness 

 to an intense white heat. Raising it to a white heat in the first 

 instance, the rock-salt train was placed in the path of its rays, 

 and a brilliant spectrum was obtained. The pile was then moved 

 into the region of obscure rays beyond the red of the spectrum. 

 Altering nothing but the strength of the current, the spiral was 

 reduced to darkness, and lowered in temperature till the deflec- 

 tion of the galvanometer fell to 1°. Our question is, " What 

 becomes of the waves which produce this deflection when new 

 ones are introduced by augmenting the temperature of the 

 spiral V 



14. Causing the spiral to pass from this state of darkness 

 through various degrees of incandescence, the following deflec- 

 tions were obtained : — 



* Phil. Trans, vol. cliv. p. 327. 



