32 FRAGMENTS OP 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 
