ELECTRIC DISCHARGE WITH THE CHLORIDE OF SILVER BATTERY. 
171 
tube ” behaves as an ordinary conductor, lend themselves as additional proof of the 
constancy of the difference of the potentials of the terminals of the tube. The 
essential part of Mr. Varley’s arrangement being a battery, of internal resistance r, 
joined by a resistance It to one terminal of a tube, the other terminal of which is 
connected to the other pole of the battery, then if P -\-n be the difference of potential 
produced by the battery, and P the constant difference of potential between the 
terminals of the tube, the current must vary as 
(P+w)-P 
ft -\-T 
or, if P be kept constant 
and large enough to render variations in r negligible, C varies as n. 
From these results recently obtained, it follows that what, in the following account 
of experiments of earlier date, is termed the “ resistance of a tube,” must not be 
considered as analogous to metallic resistance : it indicates merely that the difference 
of potential between the terminals of the tube was the same as that between the ends 
of a wire of the given resistance when substituted for the tube in the particular circuit. 
The History of some Tubes. 
No. 129, Hydrogen . 
We now give an account of the very great variety of phenomena presented by the 
same tube charged with hydrogen, No. 129, under different conditions of exhaustion 
when used in connexion with batteries of various potentials, and traversed by currents 
of different strengths. 
This tube is 32 inches long and 1*6 inch in diameter, the terminals are a bent wire 
and a ring, about 1 inch in diameter, both of aluminium ; it is furnished with a glass 
stop-cock at each end as represented in fig. 37, No. 144. The glass stop-cocks are 
connected with the mercurial pumps (Alvergniat and Sprengel) and with the gas 
generator respectively, as shown in fig. 35. 
During the course of the experiments about to be described several casualties 
occurred to the tube ; for example, the partial fusion and distortion, first of the ring, 
then of the bent wire terminal ; and by an accident the glass cocks were broken off 
and reattached. Notwithstanding these accidents, the same phenomena were again 
and again obtained under like conditions. 
The experiments with 129 were commenced on July 1, 1876 ; great precautions 
were taken to thoroughly rinse out the air by completely exhausting the tube and 
filling it with dry hydrogen several times. The hydrogen was obtained by the solution 
of very pure rod zinc, like that used for the batteries, in diluted pure sulphuric acid 
in the proportion of 1 part of acid to 10 of water. It was dried by pumice moistened 
with sulphuric acid, and then with rods of hydrate of potash. 
Tube 129, ls£ Charge of Hydrogen. 
Experiment 1. — Pressure 2 m.m. (millimetres), 2632 M (millionths of an atmo- 
z 2 
