scends that of any other terrestrial source. The
emission from the carbon points is capable of accurate analysis. To
simplify the subject, we will take the case of a platinum wire at
first slightly warmed by the current, and then gradually raised to a
white heat. When first warmed, the wire sends forth rays which have
no power on the optic nerve. They are what we call invisible rays;
and not until the temperature of the wire has reached nearly 1,000 deg.
Fahr, does it begin to glow with a faint, red light. The rays which
it emits prior to redness are all invisible rays, which can warm the
hand but cannot excite vision. When the temperature of the wire is
raised to whiteness, these dark rays not only persist, but they are
enormously augmented in intensity. They constitute about 95 per cent.
of the total radiation from the white-hot platinum wire. They make up
nearly 90 per cent. of the emission from a brilliant electric light.
You can by no means have the light of the carbons without this
invisible emission as an accompaniment. The visible radiation is, as
it were, built upon the invisible as its necessary foundation.

It is easy to illustrate the growth in intensity of these invisible
rays as the visible ones enter the radiation and augment in power. The
transparency of the elementary gases and metalloids--of oxygen,
hydrogen, nitrogen, chlorine, iodine, bromine, sulphur, phosphorus,
and even of carbon, for the invisible heat rays is extraordinary.
Dissolved in a proper vehicle, iodine cuts the visible radiation
sharply off, but allows the invisible free transmission. By
dissolving iodine in sulphur, Professor Dewar has recently added to
the number of our effectual ray-filters. The mixture may be made as
black as pitch for the visible, while remaining transparent for the
invisible rays. By such filters it is possible to detach the
invisible rays from the total radiation, and to watch their
augmentation as the light increases. Expressing the radiation from a
platinum wire when it first feels warm to the touch--when, therefore,
all its rays are invisible--by the number 1, the invisible radiation
from the same wire raised to a white heat may be 500 or more.
[Footnote: See article 'Radiation', vol. i.]

It is not, then, by the diminution or transformation of the
non-luminous emission that we obtain the luminous; the heat rays
maintain their ground as the necessary antecedents and companions of
the light rays