On Radiant Matter. n 



I have here a tube (Fig. 5) which will serve to illus- 

 trate the dependence of the phosphorescence of the glass 



on the degree of exhaustion. The two poles are at a 

 and 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 injure the vacuum. 

 Exhaustion has then been re-commenced, and the alternate 

 heating and exhaustion 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 vapour. Instantly conduction commences, 

 and the green phosphorescence 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 strati- 

 fications appear, which 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 to- 

 wards the potash tube. Now a wave of gresn 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. 

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

 fainter and the vacuum becoming non-conducting; but I 

 should detain you too long, as time is required for the ab- 

 sorption of the last traces of vapour by the potash, and 1 

 must pass on to the next subject. 



