126 Prof. H. Edlund on the Behaviour of 
tinuously in proportion as the exhaustion of the gas increases, 
and consequently opposes the passage of electricity by a 
resistance which increases with the exhaustion. The electro- 
motive force in question is no other than that of which I 
showed the presence in the voltaic arc and in the electric 
spark some years ago *. 
Although the existence of this force was at first denied by 
some physicists, who had no opportunity of examining the 
evidence which I furnished, and yet pronounced upon its 
value, yet it has been universally admitted in recent times, 
without further evidence being required. 
Since the solution of the question whether a vacuum is a 
conductor or not is of great importance, not only as a matter 
of theory, but also as rendering possible a correct explana- 
tion of many phenomena in cosmical physics, I propose to 
describe here certain new experiments, which show the cor- 
rectness of the results which I obtained in the researches to 
which I have referred. Experiments of the kind which I 
propose to describe have no doubt been already made by other 
physicists; but since they have not been so made as to fully 
show their significance, it is not superfluous to consider once 
more their importance. 
It is a well-known fact that the current of a Ruhmkorff’s 
coil is not able to leap over even a small distance between 
two electrodes melted into a glass tube, if the air contained 
in the tube is sufficiently rarefied. If, however, a little more 
air is admitted into the tube, the current easily passes from 
the one electrode to the other. The problem, then, is as 
follows : Is the reason why the current will not pass through 
a highly-exhausted atmosphere that the resistance in the 
highly-exhausted gas is too great, or that the resistance 
-opposed to the passage of the electricity from the electrodes 
into the atmosphere or in the opposite direction increases 
with the exhaustion ? 
We ought to obtain a clear and definite answer to this 
question, if we try, by means of an electromotive force, which 
isnot greater than that of the Ruhmkorff’s apparatus, to pro- 
duce directly an electric current in the tube without elec- 
trodes. If we succeed thus in producing a current in the 
highly-exhausted atmosphere, this atmosphere must be a good 
conductor; and the reason that the current of the Ruhm- 
korff’s apparatus will not pass through the tube must be 
sought in the resistance opposed by the electrodes themselves 
to the passage of the current. 
First Experiment.—A glass tube, of the form shown in the 
* Bull. d. Acad, Suéde, 1867-68; Pogg. Ann. vols. cxxxi, & cxxxiv.; 
Phil. Mag. [4] vol. xxxvil. p. 352. 
