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- 



