Mr. H. Jackson on Phosphorescence. 411 



resulting glows is an important and interesting one ; but it 

 must be left alone at present with the statement, somewhat 

 unsatisfactory it is feared, that, while there is no doubt that 

 special undulations of measurable wave-length are most 

 efficient in exciting phosphorescence in some substances, the 

 same effects can be produced, though to a less degree, by 

 vibrations which can, perhaps, be best described as undiffer- 

 entiated and irregular pulses. 



Returning to the sources of oscillations there is one other 

 source which has yet to be considered, and that is chemical 

 combination. The fact that many substances will phosphoresce 

 during and after exposure to the flame of hydrogen has 

 already been alluded to. The flame of coal-gas burnt in a 

 bunsen-burner will excite phosphorescence in many speci- 

 mens of lime ; but the effect is not strong enough to be shown 

 to an audience. 



Naturally this effect would be stronger the nearer the lime 

 was placed to the source of light. Inside the flame itself 

 would be the nearest attainable position, but then the heating 

 effect practically masks or destroys all others. In phenomena 

 such as the glow of phosphorus the temperature does not rise 

 to any very marked extent. It is possible to obtain chemical 

 combination in the presence of many bodies of a porous nature 

 without, during the early stages of the action, getting very 

 marked heating effects. The action of spongy platinum in 

 inducing the oxidation of coal-gas or alcohol- vapour may be 

 taken as a familiar illustration of the use of a porous material 

 for this purpose. 



In the case of a conducting metal it could not be expected 

 that the oscillations arising from the chemical combination 

 would cause phosphorescence even in the early stages when 

 the temperature has not risen to any extent ; but if such a 

 body as lime could be obtained in a very porous condition it 

 might, while acting as an inducer of chemical combination, 

 itself respond to the oscillations arising out of that com- 

 bination. 



This is found to be the case. A jet of unlighted coal-gas 

 allowed to play over warm porous lime produces a slight 

 phosphorescence, very faint, but quite visible in a dark room. 



By dusting easily volatile substances, such as finely-pow- 

 dered resin, over slightly heated lime the oxidisable vapour 

 is brought more closely into contact with the lime, and the 

 phenomenon of phosphorescence is made more visible. So 

 far, however, it has not been obtained with sufficient bril- 

 liancy to be shown to more than a few people at a time. 

 When the different limes that have already been experimented 



