PHOSPHORESCENCE. It has been observed that many bodies continue to emit light in a darkened room after having been exposed to sun light or some other strong source of light. This phenomenon is called phosphorescence. Although the name is evidently derived from 'phosphorus,' it should be noted that the cause of the luminos ity of phosphorus is a slow oxidation, and is not due to its having been previously exposed to light. Phosphorescence itself is a special case of fluorescence (q.v.), and eases where there are permanent chemical changes are excluded. Those substances W11 lell phosphoresce most brilliantly are the sulphides of barium, calcium, and ether alka line earths, diamonds, sugar, etc. It has been found that the phosphorescent light is often brilliantly colored. and also that the color of the light to which the body is exposed in order to ex cite phosphorescence has a very important in fluence upon the intensity of the phosphorescent light itself. it is observed that the same law connecting the exciting light and the phosphores cent light holds true as was noted in the case of fluorescence, namely, that the phosphorescent light is of a longer wave-length than the exciting light. A distinction has been drawn between phos phorescence and fluorescence from the fact that only fluid; exhibit fluorescence, while only solid bodies exhibit phosphorescence. Nearly all bodies which show fluorescence will show phosphores cence when in the solid state, as. for instance. if when liquid they are dissolved in gelatin and then dried. One of the earliest and most important in vestigations of phosphorescence was made by E. Becquerel, who made a special study of the inter val of time phosphorescence would continue after the phosphorescing' body was removed front the light. He invented an instrument known as the phosphoroseope, which enabled him to measure with the greatest exactness the small intervals of time involved in the phenomena. He observed that
many bodies in which by ordinary means of ob servation there was no trace of phosphorescence can be shown to phosphoresce for minute frac tions of a second. For instance. after exposure to sunlight, calcite shines with orange-colored light; aragonite gives a greenish light, as does lead glass also; uranium glass shines with a greenish light which lasts for about one-thirtieth of a second, although it has its maximum intensity at about one three-hundredth of a second after the sunlight has been Witlid raw n. Sapphires and ru bies give a brilliant pure red light, as do almost all the minerals which contain aluminum.
One most interesting feature of phosphores cence is that if a body is phosphorescing. as for instance a piece of paper covered with Balmain's paint, the application of heat at any one point will the phosphorescence there. This evidently furnishes a method for the study of those portions of the spectra which lie in the in fra-red region: i.e. those wave-lengths which are too long to affect our sense of sight. For if a strip of paper covered with Balinain's paint is exposed to sunlight for a short time and then taken into a darkened room and so placed that the infra-red spectrum coining from any source falls upon the paper, those places where there is radiation will be made manifest by the disap pearance of phosphorescence owing to the heating effect of the rays of the spectrum. This proc•ed tmr has been improved by Draper and Lommel, and now forms one of the best methods used in spectrum analysis. See LIGHT; FLUORESCENCE.