ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
623 
less perfect vacuum. When the body producing the special effect is brought up to the 
tube, the positive discharge caused by it stretches away towards the negative terminal, 
and the part of the positive luminosity on the positive side is depressed, owing to the 
formation of an imperfect negative dark space beneath the wire or tinfoil—a depression 
which would, if symmetrical all round the tube, cause the positive column to pass into 
the thin central column which leads up to the termination of the truncated portion. 
There is no doubt that the phenomenon observed in tubes of high vacua is substan¬ 
tially the same, except that the characteristic feebleness of the positive luminosity in 
such tubes prevents there being sufficient definition in the positive discharge caused 
by the special effect to enable the eye readily to recognise the identity of the two phe¬ 
nomena ; while, on the other hand, the greater breadth of the negative dark space in 
tubes of high vacua makes this repulsion of the positive column a more striking 
phenomenon than its analogue in tubes of lower vacua. That the above is a substan¬ 
tially accurate interpretation of the phenomenon there is no doubt, for if the special 
effect be produced by a ring of tinfoil we find the well-known truncated positive 
column; the hollow cone around it being, however, as we should expect from what 
has been said above, faint and very ill defined. 
We see then that positive special causes the positive luminosity to locate itself on 
that side of the tube along which runs the wire in connexion with the positive ter¬ 
minal. And we have also seen in the previous section that there is a constant dis¬ 
charge of negative electricity towards this positive luminosity from the surrounding 
parts of the tube. It follows, therefore, that we may expect positive special to be 
marked by the appearance of phosphorescence at the place where the positive special 
is being produced, and not, as in positive relief, on the opposite side of the tube; and 
this has been found to be the case. The thin pencil-like column of positive luminosity 
that appears along the line of the wire is accompanied by the same attendant phos¬ 
phorescence that we have discussed in the previous section. Moreover, the inside 
surface of the tube immediately beneath a piece of tinfoil connected with the positive 
terminal has been observed to lie covered with the well-known green phosphorescence, 
and even a shadow has been thrown upon it from a film of glass within the tube, the 
edge of which was over it and very near to it, and also from other small objects lying 
in similar positions. This completes the proof both that the positive special is in effect 
the creation of a virtual positive terminal on the inner surface of the tube, and also 
that where there is. such a virtual terminal or centre of instantaneous discharge the 
portions of the tube near thereto give off negative electricity to satisfy it; and with 
this negative discharge there are the usual molecular streams. And it further con¬ 
firms the view that neither the direction of the negative discharge nor that of the 
molecular streams is independent of the position of the spot from which the demand 
comes. 
The action of the positive special is of course of considerable violence. This enables 
us in a very convenient way at once to demonstrate the substantial identity of origin 
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