634 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
liood. It accordingly causes the negative electricity that is desired to be supplied 
to the first piece of tinfoil, and thereby causes an impulsive electric tension in 
the second, which causes a positive discharge within the tube beneath it, showing 
itself in positive luminosity. The negative that rushes to the first piece of tinfoil 
forms the usual blank space * beneath it, and would, in the ordinary state of things, 
cause a negative discharge within the tube. But before such discharge has got 
beyond the inchoate stage, the discharge in the tube has arrived at the place where the 
second piece of tinfoil is situated. Its arrival causes a sudden recall of the negative 
electricity that had left it previously, thus producing a sudden positive impulse at 
the first piece of tinfoil, and neutralising the tension there which would otherwise 
have caused the negative discharge and the accompanying molecular streams. Thus 
before the molecular streams can start they are revoked through the revocation of the 
negative discharge that would otherwise have caused them. 
That the above must be the explanation of the phenomenon is rendered evident by 
the consideration that the same relieving system when in a position not more favour¬ 
able 10 granting instantaneous relief to the first piece of tinfoil (i.e., when extended 
at right angles to the tube) is found to cause relief-phosphorescence. The only 
difference between the two cases is that the relief granted in the one case is sub¬ 
sequently revoked, while in the other it is not. And as we find that such revocation 
is effective in stopping relief-phosphorescence, it is clear that the streams that were to 
produce it could not have left the side of the glass beneath the first piece of tinfoil 
before the revocation came. Hence the time occupied by the positive electricity in 
passing along the tube is of a higher order of smallness than the time occupied by the 
negative discharge in passing off from the inside of the tube beneath the tinfoil. 
It may be suggested that the streams might have started, but have been stopped on 
their course by the subsequent action. No doubt, as we shall see, some small portion 
of them may have started, but certainly not all. For as the phenomena of virtual 
shadows and the bulging of the shadows of conductors within the tube shows, these 
molecules are capable of passing through the whole length or breadth of the tube, and 
probably could go much farther during the time occupied by their emission. Hence it 
is clear that they, or some portion of them, would have got across the tube, or at all 
events to a finite and considerable distance from the tinfoil, before the revocation 
came, if it did not arrive until the whole of the streams had started. Under such cir- 
* We Lave greatly felt the need of a word to describe the dark area which is formed round every 
source of negative discharge. Inastriated column it is called “Crookes’ space,” the “negative dark space,” 
or a “ stria space,” according as it is in the first, second, or a later segment (or physical unit) of the discharge, 
counting from the negative terminal. But in considering the structure of the discharge generally it is 
necessary to have a term which denotes it whatever be its position, and whether it occurs in a striated 
discharge or not. We have therefore adopted the term “ blank space.” It will be found to be as charac¬ 
teristic of a negative terminal as positive luminosity is of a positive terminal, however such terminal be 
formed. 
