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 In most of these cases the emission stops as soon as the stimulating waves are withdrawn, and in these cases the phenomenon has been called fluorescence.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