ON THE SENSITIVE STATE OP VACUUM DISCHARGES. 
643 
tubes of a sufficiently high exhaust to give virtual shadows,* so that it can hardly be 
said to have been proved to exist in tubes of ordinary exhaust. This cannot be denied, 
and we are not prepared with any equally rigid proof that it does so exist in tubes of 
low exhaust, although there are many circumstances which point to such being the 
case. In the first place, we have the fact that all the phenomena obtained with 
positive intermittence are sharper in character than the corresponding ones that are 
obtained with negative intermittence, pointing towards a less instantaneous action in 
the case of the negative discharge. This is a very well marked phenomenon, which 
will be found to have been noticed by us in our previous paper, and is manifested by 
tubes of every class of exhaust. Another circumstance pointing in the same direction 
is a peculiarity that has long been observed in the negative discharge, viz.: its prefer¬ 
ence for a surface as compared with a point of discharge. This, and its analogue, the 
possibility of bifurcating the negative current, seem to point to the discharge at a 
negative terminal being a continuous process, which is facilitated by its having a large 
number of places from which it can go on at the same timed 
These general considerations are but poor substitutes for the definite experimental 
proof which we were able to give for the case of tubes of fairly high exhaustion, and 
it is hoped that they may be supplemented at some future day. But one reason for 
this is that the contrast between the character of the two discharges is not a strongly 
marked phenomenon in low exhausts, though we have no doubt that it exists in some 
degree. The change that we shall in this section prove to take place as we pass from 
tubes of fairly high exhaust to tubes of extremely high exhaust takes place also as we 
pass from tubes of low exhaust to tubes of fairly high exhaust. In the former, the 
two kinds of discharge are not very markedly different in their duration character. 
As the exhaust increases, the positive discharge becomes more nearly instantaneous, 
and the negative discharge becomes more durational till we come to the class of 
exhaust treated of in that portion of the previous section which deals with that 
question. We shall now proceed to examine the case of tubes of very high exhaust, 
and consequently very great resistance, and show that the contrast there becomes 
very much intensified. 
The first class of experiments showing that such is the case consist of observations 
with the standard-tube. A ring of tinfoil was placed round a tube of very high 
exhaust with a negative air-spark, and a wire was taken from it to a ring of tinfoil 
round a tube of moderate exhaust carrying an independent continuous current. 
Negative effects were of course produced. A supplemental ring was placed round 
the latter tube, and when touched it was expected to give (in accordance with the 
* It must not be thought from this that these tubes were all tubes which gave phosphorescence in any 
marked degree with the continuous current. It was necessary to use an air-spark in most cases if it was 
desired to produce phosphorescent effects throughout the tube. But they were tubes in which the blank- 
space was of considerable breadth, and ought fairly to be considered tubes of high exhaust. 
t See De La Rue and Mulueb, Phil. Trans., Part I., Vol. 171, p. 108, and Plate 10, figs. 27 and 28. 
