NEWS FROM JUPITER. 
351 
any gas changes proportionately to the increase of pressure, 
until the gas is approaching the state when it is about to turn 
liquid. Now air at the sea-level has a density equal to less 
than the 900th part of the density of water ; so that if the 
pressure at the sea-level were increased 900 times, either the 
density would not increase proportionally, which would show 
that the gas was approaching the density of liquefaction, or 
else the gas would be denser than water, which must be regarded 
as utterly impossible. Or if any one is disposed, for the sake of 
argument, to assume that a gas {at ordinary temperatures) 
may be as dense as water, then we need proceed but a few steps 
farther, increasing the pressure about 18,000 times instead of 
900 times, to have the density of platinum instead of that of 
water, and no one is likely to maintain that our air could exist 
in the gaseous form with a density equalling that of the densest 
of the elements. We are still an enormous way behind the 
number of twenty-one figures mentioned above ; and in fact, if 
we supposed the pressure and density to increase continually 
to the extent implied by the number of twenty-one figures, we 
should have a density exceeding that of platinum more than 
ten thousand millions of millions of times ! 
Of course this supposition is utterly monstrous, and I have 
merely indicated it to show how difficulties crowd around us 
in any attempt to show that a resemblance exists between the 
condition of Jupiter and that of our earth. The assumptions 
I made were sufficiently moderate, be it noticed, since I simply 
regarded (i.) the air of Jupiter as composed like our own; 
(ii.) the pressure at the upper part of his cloud-layer as not 
less than the pressure far above the highest of our terrestrial 
cumulus clouds (with which alone the clouds of Jupiter are 
comparable) ; and (iii.) the depth of his cloud-layer as about 
100 miles. The first two assumptions cannot fairly be de- 
parted from to any considerable extent, without adopting the 
conclusion that the atmosphere of Jupiter is quite unlike that 
of our earth, which is precisely what I desire to maintain. The 
third is, of course, open to attack, though I apprehend that no 
one who has observed Jupiter with a good telescope will ques- 
tion its justice. But it is not at all essential to the argument 
that the assumed depth of the Jovian atmosphere should be 
even nearly so great. We do not need a third of our array of 
twenty-one figures, or even a seventh part, since no one who 
has studied the experimental researches made into the condition 
of gases and vapours can for a moment suppose that an atmo- 
sphere like ours could remain gaseous, except at an enormously 
high temperature^ at a pressure of two or three hundred atmo- 
spheres. Such a pressure would be obtained, retaining our first 
two assumptions, at a depth of about fourteen miles below the 
