SCIENTIFIC SUMMARY. 
105 
3. The shape and size of this dark space do not vary with the distance 
separating the poles, nor, except slightly, with increase of battery power ; 
nor with intensity of spark. 
He has devised many experiments to ascertain if this visible layer of 
molecular disturbance is identical with the invisible layer of molecular 
pressure, with the investigation of which he has for some years been 
occupied. 
The Electrical Radiometer is made with aluminium- vanes coated with a 
film of mica. The fly is supported by a hard steel cup, and the needle 
point is connected with a platinum terminal sealed into the glass. At the 
top of the ball another terminal is sealed in. The fly therefore can be made 
the negative pole of an induction coil. When thus connected, a halo of 
velvety violet light forms on the metallic sides of the vanes, a dark space 
separating the violet halo from the metal. At the pressure of half a millim. 
the dark space extends to the glass, and positive rotation commences. 
On continuing exhaustion the dark space further widens out, and appears 
to flatten itself against the glass, rotation becoming very rapid. 
With an aluminium cup the conveyance of these lines of force to a focus 
on the concave side was observed. At very high exhaustions the dark space 
fills the tube, and at length the tube becomes beautifully illuminated by 
greenish yellow phosphorescent light. In an apparatus constructed for 
observing the position of the focus it was found that this is at the centre of 
curvature, the molecules being projected in a direction normal to the surface 
of the pole. 
The spectrum of the green light is continuous, while that seen at lower 
exhaustions is characteristic of the residual gas. It is the same whether the 
residuum be nitrogen, hydrogen, or carbonic acid. It commences at different 
exhaustions in different gases. 
The rays exciting this phosphorescence radiate in straight lines from the 
negative pole, casting strongly defined shadows of objects in their path. The 
ordinary luminousness of vacuum tubes will follow any number of curves 
and angles. From an examination of these shadows and other experiments 
he advances the theory that the induction-spark actually illuminates the 
lines of molecular pressure caused by electrical excitement of the negative 
pole. 
The thickness of a dark space is a measure of the mean length of the path 
between successive collisions of the molecules. The extra velocity with 
which the molecules rebound from the excited negative pole keeps back the 
more slowly moving molecules which are advancing towards that pole. The 
conflict occurs at the boundary of the dark space where the luminous margin 
bears witness to the energy of the collisions. 
When the exhaustion is sufficiently high, the rebounding molecules spend 
their force on the sides of the vessel, and the production of light is the con- 
sequence of this sudden arrest of velocity. The light actually proceeds from 
the glass. The shadows are not optical but molecular, revealed by the 
luminous effect. Experiments are given to show that an actual material 
blow is given by the molecules. The stream is very sensitive to magnetic 
influence, the action of the magnet being to twist their trajectory round, in 
a direction at an angle to their free path, the twist being that of the current 
