LENARD'S RESEARCHES ON 

 PHOSPHORESCENCE 



By E. N. DA C. ANDRADE, B.Sc, Ph.D. 



In the following pages a brief account is given of the chief 

 phenomena of phosphorescence known in E. Becquerel's time 

 together with a description of the more recent work of Lenard 

 and his co-workers, whose labours have contributed largely to 

 the solution of the problems underlying the emission of light 

 by the atom or molecule. 



When ordinary bodies are heated, they begin to emit visible 

 light at a definite temperature, which is the same whatever the 

 substance may be (about 500 C.) ; it is to such radiation, due 

 to temperature alone and usually referred to as temperature 

 radiation, that KirchofPs law applies, though W. Wien {Nobel- 

 Vortrag, 191 1, p. 5) imagines that it may be possible to extend 

 the law to other radiations by an extension of the conception 

 of temperature ; he admits, however, that at present it is impos- 

 sible to state how, for example, a phosphorescent body can 

 fall into equilibrium with the radiation. In certain cases light 

 may be emitted at a temperature far below that at which 

 temperature radiation sets in ; such cases are classed together 

 as luminescence phenomena ; these are variously grouped under 

 the headings triboluminescence, lyoluminescence, crystallo- 

 luminescence, chemical luminescence and phosphorescence, 

 fluorescence and thermoluminescence. The first three names 

 are given respectively to the emission of light which takes 

 place on rubbing or breaking certain substances (a well-known 

 case being that of sugar), to the emission of light observed 

 when certain solid substances are dissolved, and to the emission 

 of light attending the crystallisation of salts — for instance, 

 sodium or potassium sulphate. It is probable that the two 

 latter cases are only examples of triboluminescence, the light 

 being attributable to the friction and breaking of the crystals 

 which take place on dissolution and crystallisation : apparently 



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