702 Scientific Proceedings, Royal Dublin Society. 
and south poles of Mars. Carbon dioxide would be the heaviest 
constituent of such an atmosphere, and would in that respect differ 
from water, which is the lightest constituent of the atmosphere of 
the Earth. It is inferred from this, that the vapour upon Mars 
cannot produce elevated clouds floating above the solid surface of the 
planet, as water does in the Harth’s atmosphere, but only low-lying 
fogs with frosts and snow; and to these, in connexion with the 
distillation of the vapour alternately towards the two poles, are 
referred the varying but frequently recurring appearances which 
observers have recorded upon that planet. 
The investigation also offers an explanation of such a gap in the 
series of chemical elements as we find upon the Harth between 
hydrogen and helium, and between helium and lithium; and it 
shows that, if the suspected intermediate elements exist, the con- 
ditions upon Jupiter are such that they may all be present in his 
atmosphere, and that some of them may be present upon the three 
other giant planets of the solar system, though not upon any 
of the group of four smaller inner planets to which the Harth 
belongs. 
By an application of the same method of investigation to 
the satellites and the minor planets of the solar system, the author 
infers that there can be no atmosphere upon any of these bodies, 
except perhaps upon the great satellite of Neptune; and with 
reference to the Sun, it is shown that the greatest size which the 
Sun can have had since it became a globe—that is, the greatest 
which is compatible with its atmosphere’s having then, as now, con- 
tained free hydrogen—is an immense sphere extending from the 
centre of the Sun out to a situation which lies between where the 
orbits of Mars and Jupiter now lie; so that from some such 
immense size as this it may have been since slowly contracting. 
Finally, the investigation leads to the conclusion that the 
molecules of the gases which have from time to time escaped 
from planets and satellites have but seldom been able to extri- 
eate themselves altogether from the solar system; and that the 
vast majority of them are now circulating in countless numbers 
round the Sun, like excessively minute independent planets. 
