LENARD'S RESEARCHES ON PHOSPHORESCENCE 63 



phosphoroid. They must be fibrous in structure in different 

 directions, as the phosphorescence is destroyed by crushing the 

 phosphoroid. To each emission band must correspond one 

 kind of centre, the various kinds functioning independently of 

 one another ; as a pure phosphoroid usually shows more than 

 one band, the same active metal and alkaline sulphide must 

 be capable of forming different kinds of centre. Again, a single 

 band in a pure phosphoroid has often three definite correspond- 

 ing bands in the exciting spectrum of the permanent phase 

 {i.e. three wave-lengths particularly capable of exciting it), so 

 that there must be secondary differences among the centres 

 which emit one band, enabling them to resonate to different 

 exciting wave-lengths. Furthermore, each centre must be 

 capable of three periods of oscillation, namely those corre- 

 sponding to the emission, the excitation, and the extinction 

 by the action of infra-red light to which reference has been 

 made in the introduction, of which details are given later. 

 The centres which Lenard hypothecates to satisfy these con- 

 ditions are of two kinds, the " momentary" and the " permanent " 

 centres. The permanent centres are systems consisting of atoms 

 of the active metal, the alkali metal and sulphur (say Ca x Cu y S z , 

 x, y, z being whole numbers) so arranged that both the metals 

 are held by the valency bands of the sulphur atom, the difference 

 between the various emission bands which are given by a pure 

 phosphoroid being conditioned by the number of valencies of the 

 active metallic atom by which the connection with the sulphur 

 atom is effected. In support of this view we have the fact that 

 the number of bands is never greater than the number of valencies 

 of the active metal, and that the different bands have widely 

 different intensities, corresponding to a greater facility of forma- 

 tion of certain bondages such as is to be expected. The different 

 excitation bands may correspond to different space arrangements 

 of the metallic atom with respect to the sulphur atom. 



The permanent process is most marked in phosphoroids 

 containing sulphur, 1 and hence the assumption is made that 



1 Hirsch (Heidelberg Dissertation, 1912) has recently prepared phosphoroids of 

 moderate duration which do not contain sulphur, an oxide or carbonate of the 

 alkali metal being substituted for the sulphide ; these have not been much studied, 

 but show that sulphur is not absolutely necessary for the production of permanent 

 bands. Phosphoroids without sulphur had, of course, been previously prepared, 

 notably by Crookes, Lecoq de Boisbaudran, and Goldstein. 



