Positive Column in Oxygen. 
563 
r — i. e. by 2r altogether — and determining the corresponding 
increases d l9 dx + d 2 to D. If dx was nearly equal to <i 2 , it 
was assumed that no alteration had occurred in the discharge 
outside the positive column to invalidate the experiment ; 
O O 
the force Y was taken to be the mean of -=- and ~,~ , and 
dx d 2 
the difference between dx and d 2 compared with either as an 
index of the experimental error. This method is useful when 
the discharge is unsteady. 
When the pressure is less than its value at which Y is 
discontinuous, dx is always nearly equal to d 2 , for the points 
whose coordinates are D, B. fall accurately upon a straight 
line. But for pressures above *8 mm. it is seldom that the 
discharge remains constant enough to permit the determi- 
nation of Y with the same precision. The chief cause of 
variation is the flickering of the discharge which at the 
higher pressures does not cover the whole of the cathode nor 
remain on the same part of it. And if the electrodes become 
oxidized the trouble increases. 
After making experiments with zinc and nearly pure silver 
electrodes and beiug much inconvenienced by their oxidation, 
I tried silver-gilt electrodes. These were much the most 
satisfactory, and as long as the gold lasted no sign of dis- 
colouration appeared on the cathode or discharge-tube. The 
anode deepened in colour, but only tarnished where the 
gold was obviously thinnest. The cathode closely fitted the 
discharge-tube whose diameter was nearly 2*4 cms. The 
diameter of the anode was 2 cms., so that it could move up 
and down the tube without touching it. 
Table I. 
v> 
Y. 
p. 
Y. 
P- 
Y. 
2-52 
1-85 
5-3 
4-5 
506 
3-28 
19-2 
9-3 
3-94 
121 
■96 
10-2 
1-48 
4-6 
1-38 
57 
•802 
20-0 
•804 
•802 
110 
192 
1-16 
•56 
6-25 
154 
1-21 
7-8 
•79 
187 
! 
The results of experiments with these electrodes are given 
in Table I., each compartment of which applies to a fresh 
quantity of oxygen. The errors in the determination of Y 
