Polytechnic Association. 917 



ammonia. For this body I am indebted to Mr. Henry How, of 

 King's College, Nova Scotia. 



All these I now show you in a series of what, from their inventor, 

 are called Geissler tubes, in which a spiral tube containing a trace of 

 nitrogen gas is surrounded by a jacket of glass filled with the substance 

 whose fluorescence we desire to show. 



A number of tubes, with various solutions, and arranged in differ- 

 ent patterns, were then exhibited. 



Among the substances that were earliest shown to possess fluorescent 

 properties was glass colored of a yellowish green with oxide of ura- 

 nium. This goes by the name of canary glass among the manufactu- 

 rers. Its fluorescent power is well shown by these Geisslen tubes, in 

 which a portion of the tube is made of this sort of glass, which is seen 

 to fluoresce with a very beautiful green light. I have placed among 

 these canary glass tubes some of ordinary glass, in which the electric 

 discharge is seen of its natural purple color. 



This fluorescence is, without doubt, due to the uranium, most of 

 whose salts fluoresce very brightly. Among these should be specially 

 noticed the acetate, the double acetate of sodium and uranium, the 

 sulphate and the double sulphates generally. 



When I hold this large bottle of uranium acetate in the blue beam 

 of the electric light, filtered through blue glass, you perceive how 

 brilliantly it lights up, while this other, filled with a salt of an almost 

 identical color (ferro cyanide of potassium), looks utterly dull and 

 dark. 



Solutions of these salts have, however, so little fluorescence that, 

 though by the use of certain accurate tests it may be distinguished, it 

 is yet difficult to develop, and in the Geisler tubes, though the con- 

 trary has been asserted by high authorities, cannot be made in the 

 faintest degree perceptible. 



In addition to the salts of uranic oxide, which fluoresce with a 

 green color, we have the platino-cyanides, which fluoresce with a great 

 variety of tints. Thus the platino cyanides of potassium and stron- 

 ium give a blue color, that of barrium a brilliant green, and that of 

 magnesium a rich red. 



Having thus noticed the nature of the exciting light, and of the 

 bodies capable of being excited to fluorescence by it, we come 

 naturally to a consideration of the nature of the fluorescent light 

 emitted. When a beam of sunlight, reflected from the mirror, A, 

 of a port lumiere placed in the window of a darkened room, is concen- 

 trated by a lens, B, and sifted from all but its blue and violet rays by 



