Traveks. — Presidential Address. 119 



tions were the vacuum tubes invented by Geissler. These 

 tubes were made of glass, the two ends presenting dilata- 

 tions into which platinum wires were fused, the tubes 

 being closed when as much as was then possible of the 

 atmospheric air within them had been exhausted. When 

 _the wires of one of these tubes were then connected with 

 the poles of an induction appai'atus a beautiful stream 

 of light traversed its interior, the colour of which varied 

 with the nature of the contents of the tube ; and many 

 of you have, no doubt, seen the exquisite luminous effects 

 resulting from the passage of an electrical discharge from 

 one wire to the other in such a tube. In this connection, 

 too, I have to mention that there are numbers of fluid and 

 solid bodies which become self-luminous under the influence 

 of particular rays of light shown in the spectrum, a pecu- 

 liarity first noticed in a form of spar called fluor-spar or 

 calcium-fluoride, from whence the phenomenon has been 

 named fluorescence ; and it has been found that this pro- 

 perty is exhibited by reason of the fluor-spar absorbing a 

 portion of tlie light-rays directed upon it. But the only rays 

 which produce this effect are the violet and ultra-violet rays of 

 the spectrum, the latter of which are wholly invisible to the 

 human eye except when passed through a glass or quartz prism, 

 and when the bright part of the spectrum has been carefully 

 shut off. But their existence at once becomes apparent when, 

 as already noted, they are projected upon some fluorescent 

 body. Experiment, moreover, has shown that the highly-re- 

 frangible rays which possess in the greatest degree the power 

 of exciting fluorescence are contained in large proportion in 

 the light emitted by a Geissler tube containing rarified nitro- 

 gen — a point to be specially noted in connection with the 

 experiments made by Eontgen to which I am about to allude, 

 and with the recent discovery of the gas named argon, to 

 which I intend to call your attention further on. The 

 means thus placed at the disposal of experimenters, and the 

 study of the phenomenon produced by the use of them, soon 

 opened up a wide range of discovery in the region of electrical 

 science. Mr. Cromwell Fleetwood Varley, so long ago as 

 1871, pointed out that the luminous cloud which appeared in a 

 Geissler tube as soon as the current had attained a certain 

 intensity v/as composed of attenuated particles of matter pro- 

 jected from the negative pole in all directions, and he showed 

 that an electro-magnet acted upon such a stream, gathering 

 it into an arch and attracting it. He found also that the 

 stream thus formed was intercepted when he placed a thin 

 plate of talc in its way, and that, whilst a luminous cloud was 

 formed on the side of the plate bombarded by it, what he 

 described as a " shadow," but which really was a space pro- 



