ON THE SENSITIVE STATE OF VACUUM DISCHARGES. 
G13 
forward by the authors of these papers. A loose skeleton-tetrahedron of copper wire 
was enclosed in a tube of high exhaust. A discharge with a positive air-spark of 
considerable length was made to pass through the tube and the finger was placed on 
the tube just opposite to the place where the tetrahedron lay. The shadow of the 
wires forming the tetrahedron was cast upon the relief-phosphorescence in precisely 
the same clear sharp way that would have been the case had the molecular streams 
proceeded from the negative terminal (Plate 28, figs. 21 and 22). A conductor was 
brought into contact with the outside of the tube exactly at the point where one of 
the angles of the tetrahedron was in contact with the inside of the tube. Instantly 
all the shadows of the wires bulged out to a breadth depending on the distance from 
the glass of the part casting the shadow. This was so precisely the counterpart of the 
experiment above referred to that it was impossible to mistake its meaning. The 
conductor permitted negative electricity to pass off from the inside of the tube (and 
therefore from the angle of the tetrahedron, which was situated there) in obedience to 
the demand for negative electricity that was created by the positive discharges 
through the tube. But the whole of the tetrahedron being metallic, a supply of 
negative electricity to one portion of it enabled the whole to act as a negative terminal, 
and hence negative electricity streamed from all portions of it in obedience to the 
general demand for it. With this negative electricity there streamed of necessity 
molecules, and these streams of molecules diverted from their course the similar streams 
that were passing by on their way to produce relief-phosphorescence, and hence came 
the bulging out of the shadows. Those parts that were most distant from the glass 
diverted the molecular streams at an earlier period of their course than the other 
parts ; and it was in the shadows of the former that the bulging was most conspicuous, 
showing that the effect w r as a true diversion or alteration of the direction of motion of 
the molecules. 
It scarcely needs further discussion to show that the above is the true explanation 
of the phenomenon. But there are one or two cognate experiments which serve to 
establish this yet more clearly. A similar experiment having been tried with other 
pieces of metallic wire with like results, it was then tried with a wire-shaped piece of 
glass. iNfi such effect followed. The glass being a non-conductor, the electricity could 
not pass from one part of it to another. The air-spark was then changed to the nega- 
* A slight peculiarity in the phenomenon ought to be here mentioned as indicative of the way in which 
intermittent discharge is obedient to local circumstances. One of the edges of the tetrahedron chanced to 
be made of two very fine parallel wires. The sharp shadow of the tetrahedron represented this perfectly, 
having a narrow green line between the two black lines which formed the shadows of the two fine wires. 
When the conductor was brought into contact with the tube, as before described, the shadows of these 
two fine wires bulged on the outside but not on the sides where they were nearest to one another, and 
thus the narrow green line was left intact. The inductive repulsion of the negative electricity in the two 
wires prevented any discharge taking place from them towards one another, and hence there were no 
molecular streams from those sides of the two wires that lay closest to one another, and there was nothing 
to impede the transverse molecular streams that sought to pass between these wires. 
MDCCCLXXX. 4 K 
