THE TRANSFORMATION OF ABSORBED RADIATION
No account of radiation would be complete without mentioning what
becomes of the radiation which bodies absorb, but a good deal of the
subject is in so uncertain a state that very little space will be
devoted to it.
+Absorbed Radiation converted into Heat.+--The most common effect of
absorbed radiation is to raise the temperature of the absorbing body,
and so cause it to re-emit long heat-waves. As the usual arrangement
is for the absorbing body to be at a lower temperature than the
radiating one, the waves given out by the absorber are longer than
those given out by the radiator, and so the net result is the
transformation of shorter waves into longer ones. But we have seen by
Prévost's theory of exchanges that radiator and absorber are
interchangeable, and therefore we see that those waves which are
emitted by the absorber and absorbed by the radiator are re-emitted by
the latter as shorter waves.
The mechanism by means of which the waves are converted into heat in
the body is still a mystery. That the waves should cause the electrons
to vibrate is perfectly clear, but how the vibrations of the electrons
are converted into those vibrations of the atoms {58} and molecules
which constitute heat is still unsolved, and the reverse process is, of
course, equally puzzling.
The heating of the body and the consequent re-emission of heat-waves is
not, however, the only process which goes on. In a large number of
substances, waves are given out under the stimulus of other waves
without any heating of the body at all. In most of these cases the
emission stops as soon as the stimulating waves are withdrawn, and in
these cases the phenomenon has been called fluorescence. The name has
been derived from fluor spar, the substance which was first observed to
exhibit this peculiar emission of waves.
A familiar example of fluorescence is provided by paraffin-oil, which
glows with a blue light when it is illuminated with ordinary sunlight
or daylight. Perhaps the easiest way to view it is to project a narrow
beam of light through the paraffin-oil contained in a glass vessel and
view the oil in a direction perpendicular to the beam. The latter will
then show up a brilliant blue.
A water solution of sulphate of quinine, made acid by a few drops of
sulphuric acid, also exhibits a blue fluorescence, while a water
solution of æsculin (made by pouring hot water over some scraps of
horse-chestnut bark) shines with a brilliant blue light.
Some lubricating oils fluoresce with a green light, as does also a
solution in water of fluorescene, named thus because of its marked
fluorescence.
A solution of chlorophyll in alcohol, which can be readily prepared by
soaking green leaves in alcohol, shows a red fluorescence; uranium
glass--the canary glass of which small vases are very frequently {59}
made--exhibits a brilliant green fluorescence, as does also crystal
uranium nitrate.
It is found, on observing the spectrum of the fluorescent light, that a
fairly small range of waves is emitted showing a well-marked maximum of
intensity at a wave-length which is characteristic of the particular
fluorescing substance.
There also seems to be a limited range of waves which can induce this
fluorescence, and this range also depends upon the fluorescing
substance. As a rule, the inducing waves are shorter in length than
the induced fluorescence, but this rule has some very marked exceptions.
The fact that only a limited range of waves produces fluorescence
explains a noticeable characteristic of the phenomenon. If the
fluorescing solutions are at all strong the fluorescence is confined to
the region close to where the light enters the solution, thus showing
that the rays which are responsible for inducing the glow become
rapidly absorbed, whereas the remainder of the light goes on
practically unabsorbed.
+Phosphorescence.+--Sometimes the emission of the induced light
continues for some time after the inducing waves are withdrawn, and
then the phenomenon is termed phosphorescence, since phosphorus emits a
continuous glow without rise of temperature.
Sometimes the glow will continue for several hours after the exciting
rays have been cut off, a good example of this being provided by
Balmain's luminous paint, which is a sulphide of calcium. With other
substances the glow will only continue for a very small fraction of a
second, so that it is impossible to {60} say where fluorescence ends
and where phosphorescence begins.
In order to determine the duration of the glow in the case of these
small times, an arrangement consisting of two rotating discs, each of
which have slits in them, is set up. Through the slits in one of them
the substance is illuminated, and through the slits in the other the
substance is observed while the light is cut off. By adjusting the
position of the discs with regard to each other the slits may be made
to follow one another after greater or shorter intervals, and so the
time of observation can be made greater or smaller after the
illumination is cut off.
All the bodies which have been observed to exhibit phosphorescence are
solid.
+Theory of Fluorescence.+--It is fairly simple to imagine a mechanism
by which fluorescence might be brought about, as we might assume a
relation between the periods of oscillation of certain types of
electron in the substance and the period of the stimulating waves.
Thus resonance might occur, and the consequent vibrations of the
electrons would start a series of secondary waves.
If, however, we assume resonance, it is difficult to see why there is a
range of wave-lengths produced and another range of wave-lengths which
may produce them. We should have expected one definite wave-length or
a few definite ones producing one or a few definite wave-lengths in the
glow, while if a whole range of waves will produce the effect it is
difficult to see why all bodies do not exhibit the phenomenon.
But the phenomenon of phosphorescence finally {61} disposes of any such
description, for the two phenomena have no sharp distinction between
them. Some substances are known in which the phosphorescence lasts for
such an extremely small fraction of a second after the stimulating
waves are withdrawn that it is difficult to know whether to call the
effect fluorescence or phosphorescence. It is probable, therefore,
that both are due to the same action. Now a wave of orange light
completes about five hundred million million vibrations in one second,
and therefore if an orange-coloured phosphoresence were to last for
only one five-hundredth of a second it would mean that the electrons
responsible for it vibrate one million million times after the stimulus
is removed. This is hardly credible, and becomes more credible when we
remember that in some phosphorescent substances the effect lasts for
many hours.
+Chemical Theory of Phosphoresence.+--It is more probable that the
stimulating rays produce an actual chemical change in the
phosphorescent substance. For instance, it is possible that the
vibrations of a certain type of electron in one kind of atom become so
violent as to detach it from the atom and the temporarily free electron
attaches itself immediately to another kind of atom.
The new arrangement may be quite stable; it is so in the action of
light on a photographic plate, but it may only be stable when the
electrons are being driven out of their original atoms, and in this
case the electrons will begin to return to their old allegiance as soon
as the stimulus is withdrawn. In the return {62} process the electrons
will naturally be agitated, and will therefore emit waves having their
characteristic period. The rate at which the return process takes
place will evidently depend upon the stability of the new arrangement.
If it is extremely unstable, the whole return may only occupy a
fraction of a second, but if it is nearly as stable as the original
arrangement the return may be extremely slow.
On this view, then, those substances will phosphoresce which have an
electron which is fairly easily detached from its atom and which will
attach itself to another atom, forming an arrangement which is less
stable than the original.
+Temperature and Phosphorescence.+--A confirmation of this chemical
view is provided by the effect of temperature on phosphorescence. The
rate of a chemical change is usually very largely increased by rise of
temperature, and further, at very low temperatures a large number of
chemical changes which take place quite readily at ordinary
temperatures do not take place at all.
Similarly at very low temperatures the action of the light may be more
or less stable. For example, Dewar cooled a fragment of
ammonium-platino-cyanide by means of liquid hydrogen, and exposed it to
a strong light. After removing the light no phosphorescence was
observed, though at ordinary temperatures a brilliant green
phosphorescence is exhibited, but on allowing the fragment to warm up
it presently glows very brightly.
A partial stability is shown by Balmain's luminous paint, for if it be
kept in the dark until it becomes quite non-luminous it will begin to
glow again for a {63} short time if warmed up in any way. By means of
this property the infra-red region of the spectrum may be made visible.
For this purpose a screen is coated with the paint, exposed to strong
sunlight, and then placed so as to receive the spectrum. The first
effect of the invisible heat rays is to make the portions of the screen
on which they fall brighter than their surroundings; but this causes
the phosphorescence to be emitted more rapidly, and soon it is all
emitted, leaving a dark region where the heat has destroyed the
phosphorescence.
On the whole, then, those substances which phosphoresce at ordinary
temperatures do so more rapidly as the temperature rises.
But Dewar has found a number of substances which phosphoresce only at
low temperatures, _e.g._ gelatine, celluloid, paraffin, ivory and horn.
This is not a fatal objection to the idea of chemical change, as some
chemical actions will only take place at low temperatures, but it is an
objection as quite a large number of substances only phosphoresce at
low temperatures, whereas there are not many chemical reactions which
will only take place there.
As a matter of fact, even if the idea of a chemical change be the true
one, it is not a very satisfactory one, as chemical changes are
undoubtedly very complicated ones, and it would be too difficult to
trace the change from the vibration of an electron to the chemical
change, and _vice-versa_.
No satisfactory theory therefore exists to account for the absorption
and the remission of the waves, whether accompanied or unaccompanied by
a rise in temperature of the absorbing body.
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