404 Mr. H. Jackson on Phosphorescence. 



ment. That the light given out is not to be ascribed to the 

 impurity has been determined by many experiments with 

 varying impurities and careful examination with the spec- 

 troscope. The further consideration of these physical and 

 chemical conditions is better left until the other two aspects 

 of the subject have been dealt with. 



If a large number of observations be made of the phos- 

 phorescent lights given out by compounds of such metals, 

 for example, as sodium, potassium, calcium, strontium and 

 barium, magnesium, and aluminium, it is hardly possible to 

 avoid coming to the conclusion that the colours of these 

 lights have a close resemblance to the colours of the lines and 

 bands seen in the various spectra of the different metals and 

 some of their compounds. Examination by the spectroscope 

 confirms this conclusion in several instances. It is not 

 suggested that the lines of the metals and the bands of their 

 compounds are reproduced in the spectra of the phosphores- 

 cent lights. What is noticeable is that the maxima of light 

 are grouped about these bands and lines, fading away from 

 them and extending to other parts so that a more or less con- 

 tinuous spectrum is seen with positions of greatest brilliancy. 

 In the case of some specimens of lime these positions are 

 well defined, and in some kinds of fluorspar the green and 

 some red bands are well seen either when the fluorspar is 

 heated or when it is excited by discharge in vacuo. The 

 questions of exact coincidence and of the shifting of the 

 positions of the maxima of brightness seen with different 

 compounds of the same metal need not be considered here. 

 The intention is only to emphasize the similarity between the 

 phosphorescent spectra of several metallic compounds and 

 the spectra of these compounds, or of the metals in them, 

 obtained in other ways. 



In experimenting with phosphorescent compounds it is 

 frequently noticed that specimens of the same substance in 

 apparently the same state of purity give different colours. 

 Confining attention for the present to lime, as a very 

 infusible substance easily obtained in a state of purity, what 

 follows will be made clearer by a brief consideration of the 

 spectrum of the coloured flame produced by holding some 

 compound of calcium, e.g. calcium chloride, in the flame of 

 a bunsen- burner. 



The spectroscope breaks this red flame up into red, 

 orange, and green bands and a blue line. For the moment 

 the suggestion may be taken that these differently coloured 

 bands are indications of the existence in the flame of groups 

 of particles of calcium compounds of varying degrees of 



