1874.] The Saturnian System. 19 
the one which has hitherto been nearly always followed. 
Very few have thought of explaining Jovian and Saturnian 
phenomena by a reference to solar phenomena; very 
many, including believers as well as disbelievers in the 
habitability of Saturn and Jupiter, have endeavoured to 
force the observed phenomena into accordance with the 
familiar phenomena of our own earth. 
We notice at once that the same low density being recog- 
nised in the case of the sun as we have been discussing in 
the case of the giant planets, any explanation which presents 
itself in the sun’s case is available, at least to be tested by 
whatever evidence may exist. 
Now we can have no hesitation in ascribing the sun’s 
small mean density to the great temperature of his whole 
globe. This temperature operates against those effects of 
pressure which, in his case, would operate far more markedly 
than in the case of the giant planets. Many elements, 
which in his interior would be solidified or liquefied by the 
great pressures to which they are subjected, remain gaseous, 
and others which do not remain gaseous yet exist at a density 
far lower than that which they would have if at ordinary 
temperatures. We may say of the sun as a whole that its 
globe is expanded, and so reduced in density by excess of 
heat. We are not able to explain precisely how his various 
elementary components are disposed throughout his globe 
in consequence of the temperatures and pressures at which 
they severally exist. In fact, everything in the sun is so 
unlike what we are familiar with, that we cannot apply 
directly any of the known laws of physics to the interpreta- 
tion of solar phenomena. But we remain, nevertheless, 
satisfied that the main reason why gravity has not its will 
throughout the solar orb, compressing the substance of that 
orb until a high mean density results, is that the great heat 
of the sun opposes the process of contraction which would 
operate at once if gravity were left unresisted. 
The inference is that the orbs of Jupiter and Saturn are 
in like manner intensely heated, though, it need hardly be 
said, by no means to the same degree. Nor, in fact, can it 
be necessary, to maintain the mean densities of Saturn and 
Jupiter at their present low amount, that anything like the 
same degree of heat should exist in their case as in the sun’s. 
For the pressures due to gravity are far less even near the 
surface of these planets, and the distance from surface to 
centre, on which the amount of the increase of pressure 
beneath the surface depends, is very much less. But that 
the temperature of Saturn and Jupiter greatly exceeds that 
