638 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
revocation, might very easily have been confounded with those of positive and negative 
discharge. These additional time-quantities are the periods required for the formation 
of the positive luminosity, and its correlative, the blank-space. These two cannot be 
considered separately, for they are really two parts of the same phenomenon. Where 
positive luminosity ends, there the blank space begins ; not that there is necessarily a 
sharp division between them (though such is ordinarily the case), for they may appear 
to shade into one another, but that they represent the two states in which the effective* 
electrical field can exist during a discharge; and these two states are mutually exclu¬ 
sive, however sharply or gradually their boundaries may change. It is the time required 
for bringing the space within the tube into one or other of these states that we are 
now about to consider. 
We are here approaching one of the most difficult problems connected with the 
elective discharge, and at the same time one that, above all others, is needful to be 
solved if we would get at the real secrets of its mechanism. In these two phenomena 
lies the most important portion of that electric dissymmetry from which we may chiefly 
hope to get light as to the nature of electricity. We do not feel that we are at present 
sufficiently advanced to treat these questions in so satisfactory a manner as we could 
wish, but we hope to be able to throw some light upon them even at the present stage. 
Taking, first, the question of positive luminosity, it is clear that its formation along 
the tube cannot be more rapid than the velocity of the discharge itself—of the positive 
discharge itself. For although no doubt static induction outruns the discharge so 
much that, as we have said, we treat its advance as absolutely instantaneous, yet the 
distance by which it effectively precedes the front of the discharge does not increase 
during its progress. It is not by the rapidity with which static induction moves that 
the impulsive character of the action of the discharge both within the tube and upon 
the surrounding space is caused, but by the rapidity with which the free electricity, 
carrying with it its static induction, is brought locally into the neighbourhood. Thus 
an inferior limit of the time required for the advance of the positive luminosity is the 
time required for the advance of the positive discharge. 
But this is only the less important part of the'matter. The real question is whether 
this luminosity is produced immediately on the discharge, or whether it is separated 
from it by an interval of time comparable with the small quantities of which we have 
been speaking. As to this we think that we must come to the conclusion that it is 
cotemporaneous with the discharge that causes it. When we get positive-special 
effects in the way to which we have so frequently referred in the present section, we 
find that we can form the typical hollow cone and truncated positive column. Now it 
is clear that this hollow cone must mark a discharge that has really taken place, either 
* We use the phrase effective electrical field to denote that part of the tube that is really affected by 
the discharge, and that undergoes the rapid alterations of electric state which accompany it. We shall 
presently see that it is quite possible for a portion of the tube to be outside this effective electrical field, 
and thus to participate in no way in the discharge. 
