LENARD'S RESEARCHES ON PHOSPHORESCENCE 55 



the bodies which exhibit the phenomena in question are all 

 triboluminescent. 1 Chemical luminescence is the form of 

 luminosity accompanying certain chemical actions, such as slow 

 oxidations : the so-called " phosphorescence " of phosphorus and 

 of putrefying organic matter are cases in point. 



The term phosphorescence is properly applied to the power 

 which many bodies possess of emitting light after excitation 

 by radiations. This excitation can be effected not only by visible 

 and invisible (ultra-violet) light but also by cathode and canal 

 rays and by Rontgen rays ; irradiation of some kind is neces- 

 sary, however, in all cases of true phosphorescence. 



In phosphorescence, the emission of light continues after 

 the exciting radiations have ceased ; if the emission does not 

 persist during a measurable time the phenomenon is termed 

 fluorescence. In the case of solids there is no true fluorescence, 

 although the term is often used in speaking of the phosphores- 

 cence of very short duration which is exhibited by many solids : 

 in the case of gases and liquids the duration of the period of 

 after-glow is inappreciable and we may speak of fluorescence. 

 But there is little point in attempting to distinguish rigidly 

 between the two terms, though it is possible that more refined 

 measurement would show a very short after-glow even in the 

 case of gases. Thermoluminescence, so-called by E. Wiedemann, 

 who first observed it, is the property of selective light-emission 

 which certain artificial substances exhibit on being heated to 

 a temperature far below that which conditions temperature 

 radiation ; it is necessary to excite the substance previously 

 by certain radiations, which do not, however, cause the emission 

 of light at ordinary temperatures. This is only a particular 

 case of phosphorescence, the exciting energy being stored at 

 the lower temperature and only liberated as the transformed 

 radiation at the higher : all the phosphoroids — as we shall in 

 future call phosphorescent solids — prepared by Lenard can 

 be caused to show such a storage of energy. Hence we shall 

 include the so-called fluorescence of solids and thermolumines- 

 cence under the general term phosphorescence. 



Some of the first observations of true phosphorescence 

 seem to have been made on gems : for instance, Boyle and 

 afterwards Wolf observed the phenomenon in the case of 



1 See Kayser, Handbuch der Spektroscopie, p. 678, where a detailed account of 

 the results of various experimenters will be found. 



