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 thermometer into the dark space beyond it.  Here he found the temperature actually higher than in any part of the visible spectrum.  By this important observation, he proved that the sun emitted heat-rays which are entirely unfit for the purposes of vision.  The subject was subsequently taken up by Seebeck, Melloni, Mueller, and others, and within the last few years it has been found capable of unexpected expansions and applications.  I have devised a method whereby the solar or electric beam can be so filtered as to detach from it, and preserve intact, this invisible ultra-red emission, while the visible and ultra-violet emissions are wholly intercepted.  We are thus enabled to operate at will upon the purely ultra-red waves.