ensity has been increased a thousand-fold by the augmentation of
temperature necessary to the production of this white light. Both
effects are bound up together: in an incandescent solid, or in a
molten solid, you cannot have the shorter waves without this
intensification of the longer ones. A sun is possible only on these
conditions; hence Sir William Herschel's discovery of the invisible
ultra-red solar emission.

The invisible heat, emitted both by dark bodies and by luminous ones,
flies through space with the velosity of light, and is called _radiant
heat_. Now, radiant heat may be made a subtle and powerful explorer of
molecular condition, and, of late years, it has given a new
significance to the act of chemical combination. Take, for example,
the air we breathe. It is a mixture of oxygen and nitrogen; and it
behaves towards radiant heat like a vacuum, being incompetent to
absorb it in any sensible degree. But permit the same two gases to
unite chemically; then, without any augmentation of the quantity of
matter, without altering the gaseous condition, without interfering in
any way with the transparency of the gas, the act of chemical union is
accompanied by an enormous diminution of its _diathermancy_, or
perviousness to radiant heat.

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; a