20 THE POPULAR SCIENCE MONTHLY. 



tion. The two poles are at a and b, and at tlie end c is a small' supple- 

 mentary 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 recommenced, 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-conducting. I now warm the potash slightly 

 and liberate a trace of aqueous vapor. 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 stratifications appear, which rapidly get narrower, until 

 the spai'k 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 

 vapor, w^hich the heat had driven off, is reabsorbed. The purple line 

 broadens out, and breaks up into fine stratifications ; these get wider, 

 and travel toward the potash-tube. Now a w^ave of green light ap- 

 pears 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 absorption of the last traces of vapor by the potash, and I 

 must pass on to the next subject. 



Radiant Matter proceeds in Straight Lines. — The radiant matter 

 whose impact on the glass causes an evolution of light, absolutely re- 

 fuses to turn a corner. Here is a V-shaped tube (Fig. C), 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 

 and 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, in order to bring out 

 the striking contrasts of color, to bend the tubes into very elaborate 

 designs. The luminosity caused by the phosphorescence of the residual 



