m, 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 towards the red, heat soon
manifests itself, augmenting as we approach the red. Of all the
colours 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
spectrum, 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 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 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 purpose of vision.

Figure 1. Spectrum of Electric Light

The diagram drawn by Professor Mueller 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 vapour 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 vapour acts upon the rays of the sun.

********************

7. Combustion by Invisible Rays.

The sun's invisible rays far transcend the visible ones in heating
power, so that