416 Molecular Physics in High Vacua, [June, 
round ; so here the indudtion-current has produced mole- 
cular excitement at the surface of the vanes forming the 
negative pole, extending up to the side of the glass. 
When the negative pole is in rapid rotation it is not 
easy to see this dark space, so I have arranged a tube in 
which the dark space will be visible to all present. The tube, 
as you will see by the diagram (Fig. 2), has a pole in the 
centre in the form of a metal disk, and other poles at each 
end. The centre pole is made negative, and the two end 
poles connected together are made the positive terminal. 
The dark space will be in the centre. When the exhaustion 
is not very great the dark space extends only a little distance 
on each side of the negative pole in the centre. When the 
exhaustion is very good, as it is in the tube before you, and 
1 turn on the coil, the dark space is seen to extend for about 
2 inches on each side of the pole. 
Here, then, we see the induction spark actually illumin- 
ating the lines of molecular pressure caused by the excite^ 
ment of the negative pole. The thickness of this dark 
space — nearly 2 inches — is the measure of the mean free 
path between successive collisions of the molecules of the 
residual gas. The extra velocity with which the negatively 
eledtrified molecules rebound from the excited pole keeps back 
the more slowly moving molecules which are advancing to- 
wards that pole. The conflict occurs at the boundary of the 
dark space, where the luminous margin bears witness to the 
energy of the discharge. 
I will endeavour to throw on the screen an illustration of 
this dark space. A stream of water falls from a small jet 
on to a horizontal plate of glass. The water spreads over the 
plate and forms a thin film. The jet of water in the centre, 
from the velocity of its fall, drives the film of water before it 
on all sides, raising it into a ring-shaped heap. As I diminish 
the force of the jet the ring contracts : this is equivalent to 
the exhaustion getting less. When I increase the force of 
water the ring expands in size, the effecft being analogous to 
an increase of exhaustion in my tubes. The extra velocity 
of the falling particles of water drive the in-coming particles 
of water before them, and raises a ridge round the side 
which exadtly represents the luminous halo to the dark space 
to be seen in this tube. 
If, instead of a flat disk, a metal cup is used for the 
negative pole, the successive appearances on exhausting 
the tube are somewhat different. The velvety violet halo 
forms over each side of the cup. On increasing the ex- 
haustion the dark space widens out, retaining almost exa( 5 tly 
