598 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
if the air-spark be considerable we often shall find phosphorescence produced in the 
test tube in the neighbourhood of the tinfoil ring that is upon it. In many cases, 
however, the effects will be less easily recognized, and recourse must be had to the 
various methods of examining the intermittence in the standard-tube described in 
Section XY. But these difficulties are common to tubes of all kinds of exhaust when 
negative effects are being examined. The effects are less sharp than those of positive 
intermittence, and we must be content with less perfect definition. The result is, 
however, sufficient to enable us to detect with certainty the existence of negative 
impulses in the tinfoil upon the standard-tube ; and reasoning as in the other case, this 
shows that the negative charges which burst into the tube pass through it in the 
shape of negative electricity. 
It must not, however, be supposed that in all cases the one discharge passes quite to 
the other terminal of the tube without exciting a response. If this were the case, 
high-tension tubes would present a uniformity of behaviour which even low-tension 
tubes do not possess. It is often very difficult to trace the evidences of the discharges 
in the immediate vicinity of the opposite terminal, and in some tubes it would seem that 
the response can come from the opposite terminal a little before the original discharge 
has reached it. But these last cases are of an exceptional character, and do not at all 
affect the conclusion that the discharge in general passes through the tube up to the 
immediate neighbourhood of the opposite terminal before exciting a response. In 
most tubes evidences of positive relief can be obtained (with a positive air-spark of 
considerable length) close up to the negative terminal. 
The evidence obtained by this method is so direct and so unmistakable in its 
signification, that it leaves no room for doubt. And just as in tubes of low exhaust 
we found that these propei’ties of the intermittent discharge rendered possible a 
variety of other phenomena, all of which were explicable by this theory of the dis¬ 
charge, so also in the case of tubes of high exhaustion all the other phenomena winch 
follow from these properties of the discharge are also manifested, and serve in their turn 
to demonstrate the identity between discharges in high-vacuum and in low-vacuum 
tubes. Thus very good unipolar and double unipolar effects can be obtained, mani¬ 
festing, with certain modifications, the same peculiar phenomena which we are 
accustomed to see in connexion with them in low-vacuum tubes. So easily are these 
effects attainable that (as will be seen later on) they afford to us the readiest way of 
obtaining luminous discharges suitable for an important part of our investigation. 
And if we recollect how intimately the existence of unipolar and double unipolar dis¬ 
charges in ordinary vacuum tubes is connected with the fact that the discharge at 
each terminal is independent of the action elsewhere than in its own neighbourhood, 
we shall see that the existence of similar phenomena in high-vacuum tubes is strong 
evidence of the substantial identity of the modus operandi of the discharge in the two 
cases. 
The various types of evidence which we have already given represent, after all, only 
