584 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
men on does not depend on the existence of any specially higli degree of exhaust. If 
a strong current be made to pass through any tube of not too poor a vacuum to be 
capable of giving a positive luminous column, and a considerable air-spark be intro¬ 
duced in the positive portion of the circuit, so as to cause the positive electricity to 
pass through the whole length of the tube in strong charges, the contact of the finger 
with the tube will, in almost all cases, cause a bright patch of phosphorescence to 
appear on the opposite side of the tube (Plate 26, fig. 8). The reason is obvious. The 
interior of the tube beneath the finger acts as a negative terminal, pro tem., to the 
advancing positive electricity, and in the act of thus sending off negative electricity it 
sends off also the streams of molecules that accompany negative discharge, and thus 
produces phosphorescence in the tube. This will be the case even in tubes which are 
full of bright luminosity, and the molecular streams will drive through this luminous 
mist without necessarily dispersing it : a phenomenon which is in itself a sufficient 
proof that they do not require a specially high degree of exhaust, since we shall find 
that this is incompatible with the existence of bright luminosity.* 
There is yet another arrangement which enables us still further to increase the 
violence of the negative discharge so as to obtain phosphorescence in tubes in which it 
does not ordinarily appear. This is the arrangement referred to in our former paper, 
on page 170 and Plate 15, fig. 2, and is due to Mr. Ward, our assistant. It consists 
in bringing a wire from the positive terminal of a Holtz machine to a small tinfoil 
patch on the outside of a tube, one of the terminals of which is in metallic connexion 
with the negative terminal of the same machine. On separating the terminals of the 
machine to a distance of, say, half-an-inch, a stream of violent sparks will of course 
pass from the one to the other. Each of these will cause, at the positive terminal, 
and therefore at the tinfoil of the tube, a sudden downfall of positive tension or rise 
of negative tension, and thus will be equivalent to an impulsive negative charge there. 
As we have seen, this will make the inside of the tube act as a negative terminal for 
the instant, and with this there will be the accompanying molecular streams, and 
phosphorescence will appear on the opposite side of the tube. Inasmuch as in this 
way we are able to attain to a much greater degree of violence in the individual dis¬ 
charges, we are by it enabled to demonstrate the existence of phosphorescence due to 
negative discharge in tubes in which all other methods fail to show it. We shall now 
give a few experiments to show the very wide range of exhaust through which, by 
some or all of the above methods, we have been able to obtain it. 
We first of all tried the tubes which were most frequently used by us in our former 
investigation, viz.: tubes of a moderate exhaust representing some 1 or 2 millims. 
* This experiment is interesting also for another reason. It shows that the action to which the 
emission of these streams is due must take place on the hounding surface of the solid and gaseous matter, 
for in this case the electricity can only come from the surface of the glass or the gaseous film in imme¬ 
diate contact with it, and not from the interior of the solid body, as is the case with the negative 
terminals of tubes. 
