226 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
the sensitive discharges. They are so sudden and yet of such considerable magnitude 
that in considering the change they effect we may proceed as though no other electrical 
forces were at work. Thus, if we take the case of the positive unipolar discharge, a 
conductor connected with the acting terminal will, if approached towards the head of 
the column, drive it back, because when the sudden rush of electricity comes to the 
terminal, and from it into the tube, there is an equally sudden rush of the same kind 
to the conductor. Each causes an impulsive change in the electric tension around it; 
and as these are of a character to nullify one another within the tube the column is 
shortened. But a Leyden jar charged with the same kind of electricity to a much 
higher potential than the source of the discharge will not drive back the column. 
All that it does is to set up a permanent electric field which offers no resistance 
whatever to the sudden impulsive electrical changes that occur in the terminal and 
in the space within the tube. 
The authors of this paper have abstained purposely from dwelling upon any 
questions involving theories of the nature of electricity or the difference between 
positive and negative electricity, or upon other differences which have come to light 
during the present investigation. But it is important to observe that the difference in 
behaviour of the two kinds of electricity is not confined to cases in which these electri¬ 
cities leave the terminals proper of the tube, but is equally present when they come 
from quasi-terminals (as in the case of the relief and non-relief-effects), or even when 
they form their own terminals out of gaseous materials, as in the case of striae. 
Postscript. 
(June 28, 1879.) 
A .—On the var iations of form of the negative glow. 
It is obviously a matter of great importance to the theory of striae put forward in 
this paper, that the negative glow should be shown to be merely a stria modified by 
the local conditions that exist at the negative terminal of the tube. We have there¬ 
fore thought it desirable to record one or two cases of special forms assumed by the 
negative glow under special circumstances, which have been met with by us since the 
date at which this paper was originally presented to the Royal Society, and which 
appear to us to show very clearly that the negative glow is only a modified stria. 
The first case was observed by one of the authors of this paper in a tube belonging 
to Mr. Warren De La Rue, which was being experimented upon in his laboratory with 
a current of several thousand cells of his well-known battery. The negative terminal 
was in the form of a rather large ring, but the exhaust of the tube was so good, and 
consequently the breadth of the Crookes’ space was so considerable, that the negative 
