164 M. W. Siemens on the Admissibility of the 
zodiacal light must have everywhere the period of revolution 
that planets would have at the distance in question from the 
sun. A resistance experienced by planets from the material 
parts of interplanetary space, moving nearly uniformly with 
them round the sun, is therefore out of the question. Only a 
resistance (here negligible) must take place in consequence 
of the inclination of their ecliptic to the plane of the sun's 
equator, to which the observed lessening of this angle of in- 
clination may perhaps be traced. The satellites, too, in re- 
volving round their planets, must experience a resistance from 
the atmosphere of cosmical space; while the extreme limit of 
the atmosphere of the planets, rotating with them, must under- 
go a frictional resistance. With respect to the moon M. Hirn 
is perhaps right in maintaining that, the celestial bodies 
moving with such prodigious velocity, even the most rarefied 
resisting medium would sweep away their atmospheres. 
Xumerous observations make it highly probable that cos- 
mical space, at least within the region of the solar system, is 
filled with combustible matter. This also indirectly tells very 
decidedly for my brother's hypothesis that the products of 
combustion, in a state of extreme rarefaction and at very low 
temperature, are again dissociated by the sun's rays. The 
objection which has been made, that the work of dissociation 
would absorb the energy of the luminous rays and thereby 
render cosmical space opaque, could be set aside by supposing 
that the work of dissociation is done by the invisible chemi- 
cally acting rays only. But it can also be assumed that, in 
the course of the ages, the work of dissociation is already ac- 
complished, and that now only the chemically combined mass 
continually emanating from the sun is still to be dissociated 
by his rays, for which only a part of the light-energy would 
be expended. 
"Without the hypothesis of dissociation it would be difficult 
to explain why cosmical space is not filled, like the earth's 
atmosphere, essentially with oxygen, nitrogen, and aqueous 
vapour. It cannot be assumed that the composition of the 
body of the sun is essentially different from that of the earth, 
if both have come out of the same rotating nebulous mass, 
since to suppose a separation of matter in the gaseous state 
according to specific gravity is inadmissible. Hence, at least 
in the solar system, the electronegative substances must every- 
where predominate ; and it is to be assumed that even the 
cold burnt-up sun in the future will be surrounded by an 
atmosphere containing an excess of oxygen. But if cosmical 
space is filled with highly rarefied dissociated products of 
combustion, these must become subject to the attraction of 
