1879. 



on Molecular Physics in High Vacua. 



153 



FiCx. 12. 



alumina — the ruby, conmdum, or artificially precipitated alumina — 

 is the same as described by Becqucrel twenty years ago. There is 

 one intense red line, a little below the fixed line B in the spectrum, 

 having a wave-length of about G895. There is a continuous spectrum 

 beginning at about B, and a few fainter lines beyond it, but they are 

 so faint in comparison with this red line that 

 they may be neglected. This line may be called 

 the characteristic line of alumina. 



I now pass on to another fact connected with 

 this negative discharge. Here is a tube (Fig. 12) 

 with a negative pole (a, 6) in the form of a 

 hemi - cylinder, similar to the one you have 

 already seen (Fig. 3), but in this case I receive 

 the rays on a phosphorescent screen (c, d). See 

 how brilliantly the lines of discharge shine out, 

 and how intensely the focal point is illuminated ; 

 it lights the whole table. Now I bring a small 

 magnet near, and move it to and fro ; the rays 

 obey the magnetic force, and the focus bends one 

 way and the other as the magnet passes it. I can 

 show this magnetic action a little more definitely. 

 Here is a long glass tube (Fig. 13), very highly 

 exhausted, with a negative pole at one end (a) 

 and a long phosphorescent screen (h, c) down 

 the centre of the tube. In front of the negative 

 pole is a plate of mica (h, d) with a hole (e) in 

 it, and the result is that when I turn on the 

 current, a line of phosphorescent light (e, /) is 

 projected along the whole length of the tube. 

 I now place beneath the tube a powerful horse- 

 shoe magnet : see how the line of light becomes 

 curved under the magnetic influence (e, ^), waving about like a 

 flexible wand as I move the magnet up and down. The action of the 

 magnet can be understood by reference to this diagram (Fig. 14). 



Fig. 13. 



The north pole gives the ray of molecules a spiral twist one way, and 

 the south pole twists it the other way ; the two poles side by side 

 compel the ray to move in a straight line up or down, along a plane 

 at right angles to the plane of the magnet and a line joining its poles. 

 Now it is of great interest to ascertain whether the law governing 

 Vol. IX. (No. 71.) m 



