640 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
as a phenomenon cotemporaneous with the negative discharge (for that is a durational 
phenomenon) in the same way that we have taken the formation of the positive 
luminosity to be cotemporaneous with the emission of the positive discharge. We 
are left then to choose between two hypotheses as to the formation of the blank-space ; 
first, that this is formed at the very initiation of the negative discharge, and that 
although the discharge itself may be durational, it is initiated sufficiently to produce 
the blank-space in a time which is no longer than that required for the positive 
discharge; or, secondly, that it does not owe its existence to actual discharge at all 
but merely to what may be called a state of readiness for discharge— i. e ., the presence 
of a quantity of negative electricity ready and eager to be discharged, but by its nature 
compelled to pass off slowly. 
These two hypotheses are not mutually exclusive, and may be, and probably are, both 
true. The best conception at which we have been able to arrive is that the blank- 
space represents a space which is sufficiently near to a source of negative discharge to 
prevent the presence in it of the peculiar action which causes positive luminosity. 
Whether or not this means that it is a space whose dimensions are such that it 
permits of some operation taking place for relieving electric tension in a gaseous 
medium—some transfusion, as it were, of the two opposing electricities that are 
gathered on its opposite sides—we are not yet prepared to assert. But its existence 
between consecutive strife and round the negative terminal point to its representing a 
space through ivhicli an operation equivalent in its results to the passage of electricity 
can take place without causing luminosity; nay, we may say that they actually demon¬ 
strate that such is the case, for in these instances the blank space actually severs the 
luminosity into segments, the luminosities in which have no connexion one with 
another. 
Leaving this question for future consideration, when our knowledge of the subject 
shall be more complete, the important conclusion to be drawn from the experiments 
last referred to is that there is no reason to regard the formation of this blank-space as 
a phenomenon connected with the emission of molecular streams. On the contrary, it 
is of a higher order of instantaneity, for while they can be affected by revocation, it 
cannot. They are, as we have seen, of a decidedly durational character, while it seems 
to belong at least to the order of instantaneity to which the passage of electricity along 
the tube belongs, inasmuch as it shapes and modifies the positive luminosity that 
attends that passage. However we form impulsive negative discharges of the proper 
type upon the side of a tube, the blank-space appears, showing that it was formed in 
time to control the positive luminosity of the discharge. 
There is a well known phenomenon which will doubtless be thought by many to 
make strongly against these conclusions. It is that if the molecular streams be turned 
aside from passing down the tube, either by means of a magnet or by shifting the 
direction of the face of the negative terminal, the positive luminosity conies up into 
closer proximity with the negative terminal, as though the molecular streams had been 
