66 Trans. Acad. Sci. of St. Louis. 
A hole of about 5 mm. diameter was made in the cop- 
per plate on the line through the two knobs. Loud sparks 
then passed. On the negative side of the plate they 
passed to the hole, keeping along the luminous positive 
ray which passed through the hole. On the positive side 
the sparks danced about in a fantastic way within the 
luminous column. A photograph of 80 spark discharges 
is shown in Fig. B, Plate XXIV. This photograph was 
obtained by replacing the large lens of the copying cam- 
era, by a small pin-hole in a sheet of tin-foil. It is evi- 
dent that the spark discharge followed the column of air 
from which the negative electrons had been drained into 
the positive terminal. 
The Leyden jars were removed from the machine, in 
order to avoid spark discharges. The brush discharges 
were photographed by means of the camera with lens, 
Fig. C, Plate XXIV. Here the positive column is seen 
to extend through the opening in the metal screen. It 
extends to the negative glow. A feeble negative inflow 
to the edge of the copper plate (a positive brush ‘‘dis- 
charge’’) is also to be seen. A comparison of this figure 
with the former one is very instructive. It shows that 
the cathode discharge is promoted by extending a chan- 
nel of conducting air to the cathode. Nevertheless a dis- 
charge is continually passing through the dark space, 
when no opening exists in the metal screen. This trans- 
fer across the dark space is then evidently by convection. 
The air molecules are overloaded with the negative parti- 
cles at the negative terminal, in the region of the negative 
glow. After passing through the dark space the negative 
particles are delivered to the metal plate, from which they 
pass to molecules in the positive column which have been 
deprived of negative particles through drainage to the 
positive terminal. 
Fig. A of Plate XXV shows a shadow made by a glass 
tube when placed in the positive column, its end facing 
the camera. The air in this shadow is not in a condition 
to conduct the discharge from the metal plate. The tube 
cuts off the means for draining into the positive termi- 
