162 M. W. Siemens on the Admissibility of the 
photosphere, keeps one of the electricities which have been sepa- 
rated by the friction, while the other is spread by convection in 
cosmical space. As it must then be also assumed that this con- 
vection extends far beyond the orbits of the planets, the body of 
the sun must be regarded as having an electric potential with 
respect to these and acting distributively upon them. I will 
not enter further upon the controversy respecting the admis- 
sibility of the theory in question. I do not fail to perceive 
the weight of many of the reasons which have been brought 
to bear against it ; but I am of the opinion that the possibility 
of the assumption of a solar electric potential, given by the 
theory, tells much in its favour, since some of the most im- 
portant terrestrial phenomena would find in it an explana- 
tion, which has hitherto been sought in vain, and since, on 
the other hand, in the present position of physical knowledge 
the finding of any other explanation of the presence of a solar 
electric potential will scarcely be possible. For, up to the 
present time, no process is known to us in which only one 
electricity is called forth. We know only separations of the 
two electricities ; and although, according to all expectation, 
with the intensely powerful mechanical and chemical actions 
which are set up at the surface of the sun's body, such separa- 
tions take place on a very large scale, they must be again 
compensated by conduction within the same ; and even if. 
there were a lasting separation of the two electricities in the 
body of the sun, action at a distance of one of them could not 
possibly take place. So long, therefore, as no new and at 
present quite unknown facts appear, my brother's convection 
theory remains inseparable from the assumption of a solar 
electric potential. 
I must not, however, omit briefly to go into the weightiest 
objection to this theory, urged against it by MM. Faye and 
Hirn. It is that the invariability of the periods of revolution 
of the planets about the sun logically excludes the admission 
that space is filled with matter — that astronomical observations 
unconditionally demand the assumption that cosmical space is 
an absolute vacuum, since, with the prodigious velocity of the 
motion of the planets, even the most highly rarefied atmo- 
sphere must bring about a measurable diminution of the 
planetary velocities, and consequently a shortening of their 
periods. This would be true, on the hypothesis that the 
atmosphere of cosmical space is at relative rest. But this 
cannot be the case if the circulation assumed by my brother 
really takes place. It must be assumed that the sun's atmo- 
sphere has approximately the same rotation-period as the body 
of the sun. Any difference in the velocities of rotation, 
