230 



NATURE 



{July 3, 1879 



b, and at the end (c) is a small supplementary tube connected 

 with the other by a narrow aperture, and containing solid caustic 

 potash. The tube has been exhausted to a very high point, and 

 the potash heated so as to drive off moisture and deteriorate the 



Fig. 5. 



vacuum. Exhaustion has then teen re-commenced, and the 

 alternate heating and exhaustion have teen repeated until the 

 tube has been brought to the sta'e in which it now appears before 

 you. When the induction spark is first turned on nothing is 



visible — the vacuum is so high that the tube is nonconducting. 

 I now warm the potash slightly, and liberate a trace of aqueous 

 vapour. Instantly conduction commences, and the green phos- 

 phorescence flashes out along the length of the tube. I continue 

 the heat, so as to drive off more gas from the potash. The green 

 gets fainter, and now a wave of cloudy luminosity sweeps over 

 the tube, and stratifications appear. These rapidly get narrower, 

 until the spark passes along the tube in the form of a narrow 

 purple line. I take the lamp away, and allow the potash to 

 cool ; as it cools, the aqueous vapour, which the heat had driven 

 off, is re-absorbed. The purple line broadens out, and breaks 

 up into fine stratifications ; these get wider, and travel towards 

 the potash tube. Now a wave of gp'een light appears on the 

 glass at the other end, sweeping on and driving the last pale 

 stratification into the potash ; and now the tube glows over its 

 whole length with the green phosphorescence. Would time 

 allow I might keep it before you, and show the green growing 

 fainter and the vacuum becoming non-conducting ; but time is 

 required for the absorption of the last traces of vapour by the 

 potash, and I must pass on to the next subject. 



This green phosphorescence is a subject that has much occu- 

 pied my thoughts, and I have striven to ascertain some of the 

 laws governing its occurrence. I soon perceived that the phos- 

 phorescence was not in the body of the tube itself, but was 

 entirely on the surface of the glass. Another peculiarity of the 

 rays producing this green phosphorescence is that they will not 

 turn a corner in the slightest degree. Here is a V-shaped tube 

 (Fig. 5), a pole being at each extremity. The pole at the right 

 side (a) being negative, you see that the whole of the right arm 

 is flooded with green light, but at the bottom it stops sharply, 

 and will not turn the corner to get into the left side. When I 

 reverse the current, and make the left pole negative, the green 

 changes to the left side, always following the negative pole, 

 leaving the positive side with scarcely any luminosity. 



In the ordinary phenomena exhibited by vacuum tubes — 

 phenomena with which we are all familiar — it is customary, for 

 the more striking illustration of their contrasts of colour, to have 

 the tubes bent into very elaborate designs. The positive lumi- 

 nosity caused by the phosphorescence of the residual gas follows 

 all the convolutions and designs into which skilful glass-blowers 



can manage to twist the glass. The negative pole being at one I best phenomena of vacuum tubes. I have here two bulbs (Fig. 6), 

 end and the positive pole at the other, the luminous phenomena alike in shape and position of poles, the only difference being 

 seem to depend more on the positive than on the negative at an that one is at an exhaustion equal to a few millimetres of mercury 

 ordinary exhaustion such as has hitherto been used to get the 1 — such a moderate exhaustion as will give stratifications or the 



