Prof. Tyndall on Luminous and Obscure Radiation. 331 



5. The hydrogen-flame was now substituted for the gas-flame ; 

 the visible spectrum disappeared, and the deflection fell to 



12°. 

 Hence, as regards rays of this particular refrangibility, the emis- 

 sion from the luminous gas-flame was two-and-a-half times that 

 from the hydrogen-flame. 



6. The pile was now moved to and fro, and the movement in 

 both directions was accompanied by a diminished deflection. 

 Twelve degrees, therefore, was the maximum deflection for the 

 hydrogen-flame ; and the position of the pile, determined pre- 

 viously by means of the luminous flame, proves that this deflec- 

 tion was produced by extra-red undulations. 1 moved the pile a 

 little forwards, so as to reduce the deflection from 12° to 4°, and 

 then, in order to ascertain the refrangibility of the rays which 

 produced this small deflection, I relighted the gas. The recti- 

 linear face of the pile was found invading the red. When the 

 pile was caused to pass successively through positions correspond- 

 ing to the various colours of the spectrum, and to its extra- violet 

 rays, no measurable deflection was produced by the hydrogen- 

 flame. 



7. I next placed the pile at some distance from the invisible 

 spectrum of the flame of hydrogen, and felt for the spectrum by 

 moving the pile to and fro. Having found it, I without diffi- 

 culty ascertained the place of maximum heating. Changing 

 nothing else, I substituted the luminous flame for the non-lumi- 

 nous one; the position of the pile when thus revealed, was 

 beyond the red. 



8. It is thus proved that the radiation from a hydrogen-flame 

 is sensibly extra-red. The other constituents of the radiation 

 are so feeble as to be thermally insensible. Hence, when a body 

 is raised to incandescence by a hydrogen-flame, the vibrating 

 periods of its atoms must be shorter than those to which the 

 radiation of the flame itself is due. 



9. The falling of the deflection from 30° to 12 c when the hy- 

 drogen-flame was substituted for the gas-flame is doubtless due 

 to the absence of all solid matter in the former. We may, how- 

 ever, introduce such matter, and thus make the radiation origi- 

 nating in the hydrogen-flame much greater than that of the gas- 

 flame. A spiral of platinum wire plunged in the former gave a 

 maximum deflection of 52° 



at a time when the maximum deflection of the gas-flame was only 



33°. 



10. It is mainly by convection that the hydrogen-flame dis- 

 perses its heat : though its temperature is higher, its sparsely- 

 scattered molecules are not able to cope, in radiant energy, with 



Z2 



