168 M. W. Siemens on the Admissibility of the 
distinguished from nonconductors only by the maximum of 
polarization of the former being vanishingly small. That 
in very highly heated gases the maximum of polarization 
would become, as in metallic conductors, vanishingly small, 
can scarcely be admitted. Direct experiments on the di- 
electric properties of highly heated flameless gases are not 
known to me ; but the phenomena of the electric spark, as 
well as the luminous appearances in the ozone-apparatus and 
Geissler tubes, and the beautiful experiments of Hittorf *, can 
be explained even without assuming that highly heated con- 
duct differently from cold gases of equal density. Hence the 
high temperature of the solar gases appears at present to be 
no obstacle to ascribing to them insulating properties. 
Indeed their maximum of polarization will, in correspondence 
with the density of the sun's atmosphere, be greater than that 
of our cold atmospheric air, notwithstanding their elevated 
temperature. 
Very different relations, however, may appear on the 
appearance of the critical state at greater depths in the 
sun. For the electrical property of the critical state we have 
neither experiments nor analogies : hence Ave can assume 
that the interior of the sun is also a metallically conducting 
mass — that is, having a vanishingly small polarization- 
maximum. The surface of this mass of the sun in the critical 
state might then have an electric potential. The question, 
however, would have to be taken into consideration whether 
the conducting photosphere might not on the face turned 
towards the interior of the sun become electric by distribution, 
so that the sun with its enveloping photosphere would form a 
* M. Hittorf, in a communication in vol. xix. of "Wiedemann's Annalen, 
p. 73, says that what I communicated to the Academy on the 9th 
November, 1882, viz. that gases at temperatures from 1500 c to 2000° C. 
still appear perfectly dark if they are quite flameless, and that the lumi- 
nosity of gases on the passing through them of an electric current is a 
similar process to the shining of a flame that separates no solid com- 
ponents, had been previously made known by himself and others. I 
willingly grant this with respect to the nonluminosity of hot gases ; but 
I made no claim to priority in this communication : I believe, however, 
that I first proved by experiment that gases so highly heated actually 
appear perfectly dark, although the hot layer of air be over a metre thick 
and the eye has been rendered in the highest degree sensitive by complete 
darkness. Hittorf 's experiments proved only that hot gases are relatively 
dark. As to the conductivity of gases, assumed by Faraday for high 
tensions, in my paper above cited, about 25 years older, I already set up 
the general law according to which the conductivity of gases commences. 
To this I might also refer M. Eilhard Wiedemann, who claims priority for 
his explanation of gases becoming luminous on an electric current passing 
through them as a result of dielectric polarization. 
