182 FRAGMENTS OF SCIENCE. 



end toward the red, heat soon manifests itself, augmenting 

 as we approach the red. Of all the colors of the visible 

 spectrum the red possesses the highest heating power. On 

 pushing the pile into the dark region beyond the red, the 

 heat, instead of vanishing, rises suddenly and enormously 

 in intensity, until at some distance beyond the red it 

 attains a maximum. Moving the pile still forward, the 

 thermal power falls, somewhat more rapidly than it rose. 

 It then gradually shades away, but for a distance beyond 

 the red greater than the length of the whole visible spec- 

 trum, signs of heat may be detected. Drawing a datum 

 line, and erecting along it perpendiculars, proportional in 

 length to the thermal intensity at the respective points, we 

 obtain the extraordinary curve, shown on the adjacent page, 

 which exhibits the distribution of heat in the spectrum of 

 the electric light. In the region of dark rays, beyond the 

 red, the curve shoots up to B, in a steep and massive peak 

 — a kind of Matterhorn of heat, which dwarfs the portion 

 of the diagram C D E, representing the luminous radiation. 

 Indeed, the idea forced upon the mind by this diagram is 

 that the light-rays are a mere insignificant appendage to 

 the heat-rays represented by the area A B C D, thrown in 

 as it were by Nature for the purposes of vision. 



The diagram drawn by Professor Muller to represent 

 the distribution of heat in the solar spectrum is not by any 

 means so striking as that just described, and the reason, 

 doubtless, is that prior to reaching the earth the solar rays 

 have to traverse our atmosphere. By the aqueous vapor 

 there diffused, the summit of the peak representing the 

 sun's invisible radiation is cut off. A similar lowering of 

 the mountain of invisible heat is observed when the rays 

 from the electric light are permitted to pass through a film 

 of water, which acts upon them as the atmospheric vapor 

 acts upon the rays of the sun. 



