pga: ] 
XV. Some Observations on the Behaviour of Electricity in 
Rarefied Air. By Prof. HE. Eptunp*. 
ROM recent observations on the passage of electricity 
through rarefied gases, the conclusion has been drawn 
that the conductivity of a gas increases with its rarefaction 
until a certain limit has been reached ; but that if this limit is 
exceeded the conductivity begins to decrease, so that at last, if 
the process of exhaustion is continued, an absolute vacuum 
must be regarded as a nonconductor. These observations have 
been generally made by using an air-pump or other apparatus 
to rarefy the air in a glass vessel, into which two electrodes of 
platinum or some other metal had been melted, and then ex- 
amining the passage of electricity through the rarefied gas. 
If we examine these experiments, which have been made at 
various times, and compare them critically with each other, 
we shall find that they by no means justify the conclusion 
hitherto drawn from them, viz. that the conductivity of gases 
attains its maximum for a certain definite rarefaction, and 
then diminishes again if the exhaustion is continued. On the 
contrary, everything goes to show that the conductivity 
increases continually up to the highest attainable limit, and 
that consequently an absolute vacuum is a good conductor. 
In fact the passage of electricity through a rarefied gas is not 
simply dependent upon the conductivity of the gas, but also 
to a considerable extent upon the greater or less ease with 
which the electricity itself effects its passage from the elec- 
trodes into the gas, or in the opposite direction. There is 
thus a resistance offered to the passage of electricity from the 
electrodes into the layers of gas in contact with them. To 
speak exactly, the experiments hitherto made show that this 
resistance increases with the exhaustion, whilst the resistance 
of the gas continually decreases. I have already endeavoured 
to show this in a previous research f. 
« The resistance which the electricity meets in its pas- 
sage from the electrodes into the gas, or in the opposite 
direction, does not consist in an electrical resistance in the 
ordinary meaning of that term, but arises from an electro- 
motive force, which acts in a direction opposed to that of the 
electric current. Direct observations, described in a former 
research {, showed that this opposed force increases con- 
* Translated from a separate impression from Exner’s Repertorium der 
Physik, communicated by the Author. 
+ Wied. Ann. vol. xv. p. 514; Phil. Mag. vol, xiii. p. 1. 
{ Ann, deChim. et de Phys, [5] vol. xxvii. p. 114; Phil. Mag. vol. xv, 
p. 1. 
