The researches which established this result also proved the elementary gases, generally, to be highly transparent to radiant heat.  This, again, led to the proof of the diathermancy of elementary liquids, like bromine, and of solutions of the solid elements sulphur, phosphorus, and iodine.  A spectrum is now before you, and you notice that the transparent bisulphide of carbon has no effect upon the colours.  Dropping into the liquid a few flakes of iodine, you see the middle of the spectrum cut away.  By augmenting the quantity of iodine, we invade the entire spectrum, and finally cut it off altogether.  Now, the iodine, which proves itself thus hostile to the light, is perfectly transparent to the ultra-red emission with which we have now to deal.  It, therefore, is to be our ray-filter.

Placing the alum-cell again in front of the electric lamp, we assure ourselves, as before, of the utter inability of the concentrated light to fire white paper-Introducing a cell containing the solution of iodine, the light is entirely cut off; and then, on removing the alum-cell, the white paper at the dark focus is instantly set on fire.  Black paper is more absorbent than white for these rays; and the consequence is, that with it the suddenness and vigour of the combustion are augmented.  Zinc is burnt up at the same place, magnesium bursts into vivid combustion, while a sheet of platinized platinum, placed at the focus, is heated to whiteness.

Looked at through a prism, the white-hot platinum yields all the colours of the spectrum.  Before impinging upon the platinum, the waves were of too slow recurrence to awaken vision; by the atoms of the platinum, these long and sluggish waves are broken up into shorter ones, being thus brought within the visual range.  At the other end of the spectrum, by the interposition of suitable substances, Professor Stokes lowered the refrangibility, so as to render the non-visual rays visual, and to this change he gave the name of Fluorescence.  Here, by the intervention of the platinum, the refrangibility is raised, so as to render the non-visual visual, and to this change I have given the name of Calorescence.

At the perfectly invisible focus where these effects are produced, the air may be as cold as ice.  Air, as already stated, does not absorb radiant heat, and is therefore not warmed by it.  Nothing could more forcibly illustrate the isolation, if I may use the term, of the luminiferous ether from the air.  The wave-motion of the one is heaped up to an extraordinary degree of intensity, without producing any sensible effect upon the other.  I may add that, with suitable precautions, the eye may be placed in a focus competent to heat platinum to vivid redness, without experiencing any damage, or the slightest sensation either of light or heat.