ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
G19 
escence. Then, again, the shadows cast by the two will often be in different directions. 
Moreover, if an object cast a clear shadow upon the relief-phosphorescence and the 
mobile portion of the phosphorescence (which we have seen belongs to the attendant 
phosphorescence) be brought up to it, we shall generally find that it blurs it so com¬ 
pletely as to render it difficult to make out its outlines, thus showing that the streams 
which give rise to it do not take their origin in the same spot as those which occasion 
the relief-phosphorescence. In short, however we examine the matter we are con¬ 
tinually met by fresh proofs of the radically distinct nature and origin of the two kinds 
of phosphorescence. 
Perhaps the most convincing experiment upon this point remains to be described. 
A tube was taken in which were a quantity of very thin films of glass lying loose in 
the tube. A spot was selected where one film lay over another with a very shallow 
interval between them. The relief-phosphorescence was thrown upon the spot of the 
tube where they were lying. It caused bright phosphorescence on the surface of the 
upper one, but left wholly unilluminated the one that lay beneath it. The positive 
luminosity with its attendant phosphorescence was now driven behind the films. 
The lower as well as the upper was now illuminated by phosphorescence. Now we 
know that the streams producing this phosphorescence are normal to the axis of the 
tube. Hence it is clear that the exciting cause of the negative discharges which 
caused these streams must have been below the upper film (and probably below the 
lower one also) since it caused streams to impinge on the lower film which could have 
come from nowhere but the lower surface of the upper film. Thus we see that we 
have the most cogent evidence for supposing that the thin column of positive lumi¬ 
nosity really represents a locus of demand for negative electricity and excites discharge 
from the sides of the tube, and possibly from the gas, to satisfy it. 
In order to establish more firmly the position that this attendant phosphorescence 
is caused by streams that come towards it in directions normal to the axis of the tube, 
and principally in directions inclined but slightly to the diameter of the normal section 
through the point, an electro-magnet was placed with its axis perpendicular to that of 
the tube and its pole near to the tube, but not near enough to produce relief-phos¬ 
phorescence. The position of the green line was carefully observed, and then the 
current through the coils of the magnet was turned on. The instantaneous effect was 
to twist the green line into the shape of an S. Now if it be remembered that, roughly 
speaking, the directions of these streams of molecules are diametral, and therefore in 
the original position were parallel to the axis of the magnet, it will be seen that the 
effect observed was precisely what should have been expected. Molecular streams 
parallel to the axis of the magnet would be bent into helices, and accordingly those 
on one side would appear to be thrown one way and those on the other would appear 
to be thrown the other way. In order to establish experimentally that this effect 
would be produced upon a system of molecular streams constituted as described, a 
broad slip of tinfoil not quite long enough to go round the tube was placed on the 
