32 FRAGMENTS OF SCIENCE. 



trum of the electric light; and for this purpose we shall 

 employ a particular form of the thermo-electric pile, 

 devised by Melloni. Its face is a rectangle, which by 

 means of movable side-pieces can be rendered as narrow as 

 desired. We can, for example, have the face of the pile 

 the tenth, the hundredth, or even the thousandth of an 

 inch in breadth. By means of an endless screw, this linear 

 thermo-electric pile may be moved through the entire 

 spectrum, from the violet to the red, the amount of heat 

 falling upon the pile at every point of its march being 

 declared by a magnetic needle associated with the pile. 



When this instrument is brought up to the violet end of 

 the spectrum of the electric light, the heat is found to be 

 insensible. As the pile is gradually moved from the violet 

 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 enor- 

 mously 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 perpendicu- 

 lars, proportional in length to the thermal intensity at the 

 respective points, we obtain the extraordinary curve, shown 

 on the opposite 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 ODE, represent- 

 ing 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 pur- 

 pose of vision. 



The diagram drawn by Professor Miiller 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 



