PEOFESSOE TYNDALL ON CALOEESCENCE. 7 



The curve representing the distribution of heat in the electric spectrum falls most 

 steeply on that side of the maximum which is most distant from the red. On both 

 sides, however, we have a continuous falling off. I have had numerous experiments made 

 to ascertain whether there is any interruption of continuity in the calorific spectrum ; 

 but all the measurements hitherto executed with artificial sources reveal a gradual and 

 continuous augmentation of heat from the point where it first becomes sensible up to 

 the maximum. Sir John Hekschel has shown that this is not the case with the radia- 

 tion from the sun when analyzed by a flint-glass prism. Permitting the solar spectrum 

 to fall upon a sheet of blackened paper, over which had been spread a wash of alcohol, 

 this eminent philosopher determined by its drying-power the heating-power of the spec- 

 trum. He found that the wet surface dried in a series of spots representing thermal 

 maxima separated from each other by spaces of comparatively feeble calorific intensity. 

 No such maxima and minima were observed in the spectrum of the electric light, nor 

 in the spectrum of a platinum wire raised to a white heat by a voltaic current. Prisms 

 and lenses of rock-salt, of crown glass, and of flint glass were employed in these cases. 

 In subsequent experiments the beam intended for analysis was caused to pass through 

 layers of water and other liquids of various thicknesses. Gases and vapours of various 

 kinds were also introduced into the path of the beam. In all cases there was a general 

 lowering of the calorific power, but the descent of the curve on both sides of the maxi- 

 mum was unbroken*. 



§3. 



The rays from an obscure source cannot compete in point of intensity with the obscure 

 rays of a luminous source. No body heated under incandescence could emit rays of an 

 intensity comparable to those of the maximum region of the electric spectrum. If, 

 therefore, we wish to produce intense calorific effects by invisible rays, we must choose 

 those emitted by an intensely luminous source. The question then arises, how are the 

 invisible calorific rays to be isolated from the visible ones. The interposition of an 

 opaque screen suffices to cut off" the visible spectrum of the electric light, and leaves us 

 the invisible calorific rays to operate upon at our pleasure. Sir William Herschel 

 experimented thus when he sought, by concentrating them, to render the invisible rays 

 of the sun visible. But to form a spectrum in which the invisible rays shall be com- 

 pletely separated from the visible ones, a narrow slit or a small aperture is necessary ; 

 and this circumstance renders the amount of heat separable by prismatic analysis very 

 limited. If we wish to ascertain what the intensely concentrated invisible rays can 

 accomplish, we must devise some other mode of detaching them from their visible com- 

 panions. We must, in fact, discover a substance which shall filter the composite radia- 

 tion of a luminous source by stopping the visible rays and allowing the invisible ones 

 free transmission. 



Could we obtain a black elementary body thoroughly homogeneous, and with all its 



* At a future day I hope to subject this question to a more severe examination. 



