90 
Messrs, w. de la rue and h. w . muller on the 
the surfaces of the positive and negative discs alone are illuminated, the glow, 
especially on the negative, having become much wider. 
The following cases of the discharge between two points in air, illustrated by the 
outline sketches A, B, C, D, E, F, fig. 82, one-tlrird of the full size, will explain 
what is meant by the arc breaking up into distinct entities ; the outlines represent 
merely the bright portions of the arc : these are always surrounded with a less bright 
portion of highly heated gas, as shown in Plate 8, figs. 1 to 33 inclusive. 
Distance of terminals, two brass points, 6 inches=152 m.m. in air, 11,000 cells. 
Fig. 
Pressure. 
Current. 
m.m. 
M. 
W. 
A 
36 
47,369 
0-02456 
B 
14 
18,421 
0-02456 
C 
14 
18,421 
0-02209 
D 
7 
9211 
0-02634 
E 
7 
9211 
not observed 
F 
56 
73,684 
0-01575 
The following experiments in air were made with the points still 6 inches distant:— 
Pressure 63 m.m., 82,895 M. Two wires, O'OOl inch diameter, had been previously 
fixed in the direction the arc would take, one at 15 m.m. the other at 80 m.m, from 
the positive terminal (F, fig. 82); both instantaneously fused and served as new inter¬ 
mediate terminals ; the heat appeared to be greatest at the wire most distant 80 m.m. 
The mean increase of temperature in the jar was at the rate of 0°'485 per second, 
and the cooling at the rate of 0°T4, so that the real increase would have been 
0‘485 + 0T4 = 0‘' - 625 supposing no cooling had occurred. 
Pressure 8 m.m., 10,526 M, 1 1,000 cells, current 0 - 0177l W. On making connexion 
the mercury in the gauge was depressed 2 m.m., so that the volume, or rather the 
pressure, increased in the ratio 1 "25 to 1. The appearance of the arc is represented 
in Plate 8, fig. 33 ; the pale portion surrounding the brighter part was quite invisible 
in subdued daylight. By keeping the current on for about two minutes the air in the 
bell-jar rose from 17° C. to 95° C. 
Pressure 67 m.m., 88,158 M, 11,000 cells, current not measurable with the galvano¬ 
meter, depression on connecting 22"2 m.m., increase of volume (pressure) in the ratio 
of P331 to 1 ; the arc is shown in Plate 8, fig. 32. Even at a pressure of 100 m.m., 
139,474 M, there was a glow visible on both the terminals. 
Pressure 8 m.m., 10,526 M, 11,000 cells, current 0'02634 W, depression on con¬ 
necting 2'5 m.m., increased pressure as 1'312 to 1. In this experiment wires were 
placed across the arc ; surrounding that one 15 m.m. distant from the positive was a 
cylindrical brighter glow resembling the glow around a negative terminal; the arc was 
crossed with close paper-like strata, as in Plate 9, fig. 9. 
Pressure 6 m.m., 7895 M, 11,000 cells, current 0‘03138 W, depression 1‘5 m.m., 
