CHAPTER VI

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 In most of these cases the emission stops as soon as the stimulating waves are withdrawn, and in these cases the phenomenon has been called fluorescence.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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