422 Proceedings of Royal Society of Edinburgh. [sess. 
pressure. One of the tubes was then sealed up ; the other was 
closed by a good fitting and well -greased glass stopcock. 
The vacuums up to 2 mins, pressure were obtained by means of 
a double-barrelled air-pump. Higher vacuums were obtained by 
means of a Topler pump. 
To observe the conductance-zero the uranium was connected to 
the insulated terminal of the electrometer, and the zinc cylinder 
to sheaths. In the following table, the results obtained in air, 
hydrogen, and oxygen are given : — 
Difference of potential between the uranium -conductance - 
zero and metallic zero. 
Pressure 
in nims. 
Hydrogen. 
Oxygen. 
Air. 
760 
+ *17 of a volt, 
(in about a min. ) 
4- ’105 of a volt, 
(in about a min. ) 
+ ’ll of a volt, 
(in about a min.) 
193 
-I- ’12 of a volt, 
(in about a min.) 
66 
+ *05 of a volt. 
(6 min.) 
+ '11 of a volt. 
(3 min.) 
8 
2 
+ *04 of a volt. 
(8 min.) 
+ ’10 of a volt in 27 
min. 
^ i 
^ 1 oF(T 
+ ’05 of a volt in 
28 min. 
The uranium -conductance -zero between mutually insulated 
uranium and zinc differs much less from the metallic zero than 
in our previous experiments. This is probably due to the oxida- 
tion of the zinc of the zinc cylinder. The conductance-zero, 
however, it will be noticed, is approximately the same in all 
three gases. 
§ 31. Leakage in air at ordinary pressure at different voltages. 
We used in our first experiments the two Leydens method 
described in § 13. The Leyden B, whose external coating was 
connected to the insulated terminal of the electrometer, and its 
internal coating to the internal coating of A, was the paraffin 
Leyden described in § 13. The Leyden A was a cylinder of 
aluminium, with one end closed with aluminium. This formed 
the external coating. The internal coating was a disc of alumi- 
nium insulated in paraffin. The uranium was placed inside a 
cardboard cylinder, with one end open and the other covered 
with aluminium so as to touch the aluminium (see diagram 8). 
