412 Mr. H. Jackson on Phosphorescence. 



with are subjected to oscillations from this chemical source 

 they yield their respective colours in the same way as before. 

 The lime which showed a green glow in the vacuum-tube or 

 when dusted on to a hot plate after exposure to the jar- 

 spark gives a green glow with the powdered resin. So also 

 in the cases of the orange and blue yielding limes. The 

 possibility of the phosphorescence being due to the resin 

 vapour itself is excluded by control experiments with other 

 porous bodies which do not phosphoresce, but yet are equally 

 active in bringing about oxidation. 



This phosphorescence was often well seen when some of 

 the limes were being prepared in a furnace. (It has been 

 already mentioned that many substances retain the power of 

 phosphorescing at a high temperature, especially if they are 

 in a very fine state of division or not quite pure.) Most of 

 the limes were made from organic salts of calcium, and as 

 the organic matter burnt away a thin and scarcely visible 

 flame played over the surface of the lirne at the top of the 

 crucible in which the calcination was carried out. It was 

 frequently quite possible to predict by watching the glow 

 which was developed in the lime, what colour would be given 

 when the phosphorescence was brought about by oscillations 

 from the other sources such as the jar-spark or the discharge 

 in vacuo. 



No one who has spent much time in experimenting with 

 various substitutes for lime in lantern work can have failed 

 to be struck by the very different appearances of the light 

 on the screen given by such bodies as magnesia and zirconia 

 in comparison with lime ; but perhaps the best examples are 

 the two mantles in use at the present day for incandescent 

 gas lights. One of them, the Welsbach mantle, gives a light 

 of almost a white colour. The other, or (Sunlight mantle, 

 shows a much pinker colour to the eye. 



Experiments with many substances used in a similar way 

 to the mantles seem to indicate that in addition to the 

 ordinary heating effect of the gas flame there is another and 

 a phosphorescent effect which probably, so far as observation 

 can tell, precedes the ordinary hot stage. It is not usual to 

 find any pure substances capable of showing this phenomenon 

 to any marked extent, unless, as mentioned just now, they 

 are in an extremely fine state of division ; a condition which, 

 like the presence of impurities, may be considered to be un- 

 favourable to the too rapid discharge of the strained particles ; 

 thus giving them the opportunity of becoming fully enough 

 charged to make their oscillations, when they are discharged, 

 of sufficient vigour to be sensibly visible. 



