i §79* Molecular Physics in High Vacua. 419 
Exhaustion has then 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- 
dudfing. 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. 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 to- 
wards 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 phosphorescence. 
Would time allow I might keep it before you, and show the 
green growing fainter and the vacuum becoming non-con- 
ducfting ; but time is required for the absorption of the last 
traces of vapour by the potash, and 1 must pass on to the 
next subject. 
This green phosphorescence is a subjedl that has much 
occupied my thoughts, and I have striven to ascertain some 
of the laws governing its occurrence. 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 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 luminosity caused by the phosphorescence of the 
