MR. W. CROOKES OX MOLECULAR PHYSICS IX HIGH VACUA. 
643 
to the impact of molecule against molecule, producing phosphorescence of the gas in 
the same way as the impact of molecules against German glass produces phospho¬ 
rescence of the glass. 
590. The following experiments were commenced at the suggestion of Professor 
Maxwell :— 
A tube was made as shown in fig. 1. The terminal a is a rectangular plate of 
aluminium, folded as shown in section fig. 2; the other terminal b is a flat disk of alumi¬ 
nium set obliquely to the axis of the tube. In front of the pole b is fixed a screen of 
mica, with a small hole in it, as shown at c ; this hole is not in the axis of the tube, 
but a little to one side of it, so that rays starting normally from the centre of the 
pole b may pass through it and strike the glass at d, whilst at the same time rays 
passing direct between the poles a and b can also pass through the hole. 
Fig. I. 
The questions which this apparatus was to answer are:—(1) Will there be molecular 
projections from the negative pole, a, in two series of plane strata normal to the sides 
of the individual furrows, or will the projection be perpendicular to the electrode as a 
whole, i.e., along the axis of the tube? and (2), Will the molecular rays from tire pole b, 
when it is made negative, issue through the aperture of the screen, along the axis of 
the tube, i.e., direct to the positive pole, or will they leave the pole normal to its surface 
and strike the glass as shown at d ? 
591. The tube was exhausted and connected with an induction coil; the following 
results were obtained :—At a moderate exhaustion, the corrugated pole being made 
negative, the dark space entirely surrounds it, slight indentations being visible opposite 
each hollow, where there also is a linear concentration of blue light. The appearance 
is in section as shown in fig. 2. At higher exhaustions the luminous margin disappears 
and the rays which previously formed the blue foci are now projected on the inner 
surface of the tube, where they make themselves evident in green phosphorescent light 
