144 Mr. William Crookes [April 4, 



been re-commenced, and the alternate heating and exhaustion have 

 been repeated until the tube has been brought to the state 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 non-con- 

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

 aqueous va]30ur. Instantly conduction commences, and the green 

 phosphorescence flashes out along the length of the tube, I con- 

 tinue 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- 



FiG. 4. 



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 green 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 phos- 

 phorescence. Would time allow I might keep it before you, and 

 show the green growing fainter and the vacuum becoming non-con- 

 ducting ; 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 occupied 

 my thoughts, and I have striven to ascertain some of the laws govern- 

 ing its occuiTence. I soon perceived that the phosphorescence 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 ex- 

 tremity. 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 vacuiun tubes — phe- 

 nomena 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 luminosity 



