16 THE 1896 ANNUAL ADDEESS. 



the exhaustion is rendered higher and higher, the inner dark 

 space getsAvider and wider, until at a sufficiently high exhaus- 

 tion it fills the whole tube or bulb. Mr. Crookes has worked 

 more especially at this subject, and, indeed, the tubes which 

 are now used for the production of the Rontgen rays are 

 generally called " Crookes tubes." I have seen in some of the 

 foreign periodicals the word " Crookes " used to signify one of 

 these tubes. Mr. Crookes's researches iii very high vacua led 

 him up to that most remarkable instrument, the Radiometer, 

 the nature of which led us to form clearer conceptions, than 

 we had hitherto done, of the nature of the motion of molecules 

 in gas, or rather, when the theory of the Radiometer was 

 made out, presented us as I may say with a visible exhibition 

 of the thing in actual working. 



Now these researches, which led Mr. Crookes to improve 

 his vacuum, naturally led him to examine the electrical 

 phenomena produced by excessively high vacua. 



I have said that it was with the second or inner dark 

 space that I had chiefly to do. When the exhaustion is 

 sufficient, that fills the whole tube. 



Now what takes place in this dark space ? Suppose we 

 interpose a screen, such as a plate of mica with a hole in it. 

 A portion of the discharge from the negative electrode goes 

 through that hole and continues onwards in a straight course 

 until it reaches the wall of the tube. When it reaches the 

 wall of the tube (I will suppose the tube, as it is called, to 

 be made of German glass) it produces a greenish-yellow 

 fluorescence, or phosphorescence, of very brief duration. I 

 need hardly say that if you do not limit what comes from the 

 negative electrode by the screen with a hole in it, you get 

 a broader beam which affects the glass wall over a larger 

 space. 



Now what is it that proceeds from the negative electrode 

 towards the glass, and, when it gets there, produces this 

 phosphorescence ? Is it light, or is it matter ? 



One remarkable circumstance connected with this somethincr 

 is, that you can deflect it in its course by a magnet. If you 

 present a magnet to a ray of light it does not deflect it at 

 all; but this something is easily deflected by a magnet, even 

 by a tolerably weak magnet. Mr. Crookes found, that in 

 addition to that property, if this discharge of a something 

 fell upon one side of a very light fim, formed of thin, split 

 mica, and delicately mounted so as to enable it to s})in 

 readily, it sent it spinning round ; and he believed that the 



