630 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
charge that was created by their arrival at the second piece of tinfoil, and to leave the 
corresponding negative electricity free to meet the advancing positive discharge before 
that discharge had time to come up. And similar results were obtained, mutatis 
mutandis, when the intermittence was of a negative type. 
We have no means of comparing directly the velocities of positive and negative 
electricity in a tube. But in some experiments* which we described in our previous 
paper, in which the discharge was due to the action of a small coil, it was found that 
when all the circumstances were alike at both terminals the two electricities met 
about the middle, and that the neutral zone was situated there. Further, it was 
found that the neutral zone could be shifted toward one end or the other by the use 
of a thunder plate or small condenser which was hung on the terminal, the discharge 
from which it was desired to retard. These phenomena point clearly to the theory 
that the velocities of the two electricities are the same in such a tube, and we have seen 
no phenomena which would lead us to imagine that such is not the case generally.! 
At all events, hi the absence of any evidence to the contrary, the most probable view 
is that their velocities do not differ largely, or in other words, that the times they re¬ 
spectively take to pass along the tube are small quantities of a like order of magnitude. 
The next question is the comparison of the times that the discharges take to 
pass along the tube with the times that are occupied by their emission from the ter¬ 
minals. And here we are met by a difficulty in defining what is meant by the period 
of emission of a discharge. It may well be that- if the discharge contains only a cer¬ 
tain quantity its emission may be very rapid, while if it is greatly larger in quantity 
it may be durational, and the degree of exhaust may similarly affect it.| Something 
of the sort is pointed at by the contrast between the experiment last referred to and 
those about which we are about to speak. The former seems to point to considerable 
equality between the two electricities both as to velocity and rapidity of discharge. 
The latter will be shown to point to great inequality in rapidity of discharge, the 
positive having by far the advantage. The difference may probably be accounted for 
by the fact that in the former case the discharges were of a very gentle character and 
in a tube of low exhaust, while in the experiments to which we are about to refer the 
discharges are sufficiently violent to produce phosphorescence. 
A further question arises here as to the time occupied by molecular streams in 
* Phil. Trans., 1879, p. 210. 
f It is true that the experiment has only been made in tubes of low exhaust, and that it is not safe to 
conclude that relations which exist in low exhausts between the properties of positive and negative dis¬ 
charges will hold good also in high exhausts. For example, we shall hereafter see that the degree of 
exhaust has a marked effect on their relative rates of emission. But in the present case we have direct 
experimental evidence that the velocity of negative electricity along the tube does not participate in this 
change, but that, on the contrary, negative electricity in tubes of high exhaust continues to behave as 
though its velocity were the same as that of positive electricity, or at least of the same order of magnitude 
as it. Hence we shall take such to be the case. 
J We shall return to this question in the next section. 
