108 MESSRS. W. DE LA RUE ART) H. W. MULLER OX THE 
retreat towards the positive. In the course of our experiments we have frequently 
observed the stratification extending quite up to the negative, and after a certain time 
only filling about half of the tube, as, for example, in tube 128, containing hydrogen, 
the discharge in which, with 2400 cells, the current being 0'01639 W, is shown in 
Plate 9, fig. 4, copied from a photograph obtained in 5 seconds. 
It will have been noticed that the extent of illumination of the negative in 
tube 199 gradually increased as the exhaustion and the current became greater; 
it would seem that the negative discharge requires a greater extent of outlet than 
the positive,, especially as the current increases; this is well exemplified by tube 51 
containing hydrogen ; it is 7 inches long and 2 inches in diameter; it contains a small 
ring terminal half an inch in diameter, the other being a spiral supported on three glass 
rods, and formed of a wire 19 inches long, and making four turns. When connected 
with a battery of 1200 cells, the spiral being negative, it became illuminated to a 
small extent near the positive, then the luminosity extended backwards as the current 
was increased, but not in the same ratio. 
Through a 
resistance of 
Illumination of negative. 
Current. 
ohms. 
inches. 
ratios. 
W. 
ratios. 
500,000 
2-00 
1-00 
0 00238 
1-00 
400,000 
2-50 
1-25 
0-00263 
1-10 
300,000 
4-50 
2-25 
0-00277 
1-16 
200,000 
8-00 
4-00 
0-00347 
1-34 
100,000 
9-00 
4-50 
0-00555 
2-33 
50,000 
13-50 
6-25 
0-01023 
4 29 
40,000 
15-75 
7-87 
0-01158 
4-86 
20,000 
19-00 
9-50 
0-01575 
6-62 
» { 
19-00 
more brilliantly 
1' 9-50 
0-03138 
13-18 
The resistance of the tube was found by substitution to be 10,500 ohms. 
Plate 10, fig. 27,represents the appearance with 500,000 ohms resistance in circuit; 
it is copied from a photograph obtained in 10 seconds ; fig. 28, the appearance without 
resistance, from a photograph in 5 seconds. 
The experiments described in Part III. lead to the following conclusions :— 
1. For all gases there is a pressure which offers the least resistance to the passage of 
an electric discharge. After the minimum has been reached, the resistance 
to a discharge rapidly increases as the pressure of the medium decreases. 
With hydrogen the minimum is 0'64 m.m., 842 M; at 0 , 002m.m., 3 M, it is 
as great as at 35 m.m., 46,000 IV!. 
2. There is neither condensation nor dilatation of a gaseous medium in contiguity 
with charged terminals. 
