nd 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.

The important part played by these ultra-red rays in Nature may be
thus illustrated: I remove the iodine filter, and concentrate the
total beam upon a test tube containing water. It immediately begins to
splutter, and in a minute or two it _boils_. What boils it? Placing
the alum solution in front of the lamp, the boiling instantly ceases.
Now, the alum is pervious to all the luminous rays; hence it cannot be
these rays that caused the boiling. I now introduce the iodine, and
remove the alum: vigorous ebullition immediately recommences at the
invisible focus. So that we here fix upon the invisible ultra-red rays
the heating of the water.

We are thus enabled to understand the momentous part played by these
rays in Nature. It is to them that we owe the warming and the
consequent evaporation of the tropical ocean; it is to them,