diamond was placed in the focus and anxiously watched. On
a sudden Sir H. Davy observed the diamond to burn visibly, and when
removed from the focus it was found to be in a state of active and
rapid combustion.'

The combustion of the diamond had never been effected by radiant heat
from a terrestrial source. I tried to accomplish this before crossing
the Atlantic, and succeeded in doing so. The small diamond now in my
hand is held by a loop of platinum wire. To protect it as far as
possible from air currents, and also to concentrate the heat upon it,
it is surrounded by a hood of sheet platinum. Bringing a jar of oxygen
underneath, I cause the focus of the electric beam to fall upon the
diamond. A small fraction of the time expended in the experiment
described by Faraday suffices to raise the diamond to a brilliant red.
Plunging it then into the oxygen, it glows like a little white star;
and it would continue to burn and glow until wholly consumed. The
focus can also be made to fall upon the diamond in oxygen, as in the
Florentine experiment: the result is the same. It was simply to secure
more complete mastery over the position of the focus, so as to cause
it to fall accurately upon the diamond, that the mode of experiment
here described was resorted to.

Sec. 5. _Ultra-red Rays: Calorescence_.

In the path of the beam issuing from our lamp I now place a cell with
glass sides containing a solution of alum. All the _light_ of the beam
passes through this solution. This light is received on a powerfully
converging mirror silvered in front, and brought to a focus by the
mirror. You can see the conical beam of reflected light tracking
itself through the dust of the room. A scrap of white paper placed at
the focus shines there with dazzling brightness, but it is not even
charred. On removing the alum cell, however, the paper instantly
inflames. There must, therefore, be something in this beam besides its
light. The _light_ is not absorbed by the white paper, and therefore
does not burn the paper; but there is something over and above the
light which _is_ absorbed, and which provokes combustion. What is this
something?

In the year 1800 Sir William Herschel passed a thermometer through
the various colours of the solar spectrum, and marked the rise of
temperature corresponding to each colour. He found the heating effect
to augment from the violet to the red; he did not, however, stop at
the red, but pushed his thermom