252 



NATURE 



\^yuly lo, 1879 



sent me by Me?srs. IToplun and Williams ; by ordinary li^ht it 

 is a perfectly white powder. It is just possible that the rich 

 fire of the ruby, which has caused it to be so prized, may be due 

 not entirely to the colouring-matter, but to its wonderful power 

 of phosphorescing with a deep red colour, not only under the 

 electric discharge in a vacuum, but when- 

 ever exposed to a strong light. 



The spectrum of the red light emitted 

 by all these varieties of alumina -the ruby, 

 corundum, or artificially precipitated alu- 

 mina — is the same as described by 

 Becquerel 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 6S95. 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, h) 

 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, 



J, I and how intensely the focal point is illu • 



m^^ minated ; it lights the whole table. Now 



JmlMo^^ I bring a small magnet near, and move it 



mH^^I^^^ to and fro ; the rays obey the magnetic 



^^^^^^^^^ force, and the focus bends one way and 



PjQ 12. t'ls other as the magnet passes it. I can 



show this magnetic action a little more 

 definitely. Here is a long glass tube (P^ig. 13), very highly ex- 

 hausted, with a negative pole at one end (rt) and a long phos- 

 phorescent screen (/', <■) down the centre of the tube. In front 

 of the negative pole is a plate of mica (/', d) with a hole (;■) in it, 



/■^ A 



the south pole twists it the ether 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 the magnetic deflection of the trajectory of the mole- 

 cules is the same as has been found to hold good at a lower 

 vacuum. The former experiment was with a very higli 

 vacuum. This is a tube with a low vacuum (Fig. 15). On 

 passing the induction spark it passes as a narrow line of violet 

 light joining the two poles. Underneath I have a powerful 

 electro-magnet. I make contact with the magnet, and the line 

 of light dips in the centre towards the magnet. I reverse Ihe 

 poles, and the line is driven up to the top of the tube. Notice 

 the difference between the two phenomena. Here the action is 

 temporary. The dip takes place under the magnetic influence ; 

 the line of discharge then rises, and pursues its path to the posi- 

 tive pole. In the high exhaustion, however, after the ray of 

 light had dipped to the magnet it did not recover itself, but con- 

 tinued its path in the altered direction. 



During these experiments another property of this molecular 

 discharge has made itself very evident, although I have not yet 

 drawn attention to it. The glass gets very warm where the grfeeu 

 phosphorescence is strongest. Tlie molecular focus on the tube, 

 which we have just seen (Fig. 12) would be intensely hot, and I 

 have prepared an apparatus by which this heat at the focus can 

 be intensified and rendered visible to all present. This small 

 tube {a) (Fig. 16) is furnished with a negative pole in the form 

 of a cup (1^). The rays will therefore be projected to a focus in 

 the middle of the tube (Fig. 17, a). At the side of the tube is a 

 small electro-magnet, which I can set in action by touching a 

 key, and the focus is then drawn to the side of the glass tube 

 (Fig. 17, d). To show the first action of the heat I have coated 

 the tube with wax. I will put the apparatus in front of the 

 electric lantern (</), and throw a magnified image of the tube on 

 the screen. The coil is now at work, and the focus of molecular 

 rays is projected along the tube. I turn the magnetism on, and 

 draw the focus on the side of the glass. The first thing you see 



Fig. 13. 



and the result is that when I turn on the current, a line of phos- 

 phorescent 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 



Fig. 14. 



magnetic influence (c, g), 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). The 

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



is a small circular patch melted in the coating of wax. The 

 glass soon begins to disintegrate, and cracks are shooting star- 

 wise from the centre of heat. The glass is softening. Now the 

 atmospheric pressure forces it in, and now it melts. A hole (i-) 

 is perforated in the middle, the air rushes in, and the experiment 

 is at an end. 



Instead of drawing the focus to the side of the glass with a 

 magnet, I will take another tube (Fig. 18), and allow the focus 

 from the cup-shaped negative pole (a) to play on a piece of 

 platinum wire (i) which is supported in the centre of the bulb. 

 The platinum wire not only gets white-hot, but you can see 

 sparks coming from it on all sides, showing that it is actually 

 melting. 



Here is another tube, but instead of platinum I have put in 

 the focus that beautiful alloy of platinum and iridium which Mr. 

 Matthey has brought to such perfection, and I think that I shall 

 succeed in even melting that. I first turn on the induction-coil 

 slightly, so as not to bring out its full power. The focus is now 

 playing on the iridioplatinum, raising it to a white heat. I 

 bring a small magnet near, and you see I can deflect the focus of 

 heat just as I did the luminous focus in the other tube. By 

 shiftmg the magnet I can drive the foais up and down, or draw 

 it completely away from the metal, and render it non-luminous. 

 I withdraw the magnet, and let the molecules have full play 

 again ; the metal is now white-hot. I increase the intensity of 

 the spark. The metal glows with almost insupportable brilliancy, 

 and at last melts. 



There is still another property of this molecular discharge, 

 and it is this : — You have seen that the molecules are driven 



