thousands of miles thick? How can aeriform matter withstand such a
pressure?" Very readily. It has been proved that, even when the
heat generated by compression is allowed to escape, some gases
remain uncondensible by any force we can produce. An unsuccessful
attempt lately made in Vienna to liquify oxygen, clearly shows this
enormous resistance. The steel piston employed was literally
shortened by the pressure used; and yet the gas remained
unliquified! If, then, the expansive force is thus immense when the
heat evolved is dissipated, what must it be when that heat is in
great measure detained, as in the case we are considering? Indeed
the experiences of M. Cagniard de Latour have shown that gases may,
under pressure, acquire the density of liquids while retaining the
aeriform state, provided the temperature continues extremely high.
In such a case, every addition to the heat is an addition to the
repulsive power of the atoms: the increased pressure itself
generates an increased ability to resist; and this remains true to
whatever extent the compression is carried. Indeed it is a
corollary from the persistence of force that if, under increasing
pressure, a gas retains all the heat evolved, its resisting force
is _absolutely unlimited_. Hence the internal planetary structure
we have described is as physically stable a one as that commonly
assumed."

Had this paragraph, and the subsequent paragraphs, been written five
years later, when Prof. Andrews had published an account of his
researches, the propositions they contain, while rendered more specific
and at the same time more defensible, would perhaps have been freed from
the erroneous implication that the internal structure indicated is an
universal one. Let us, while guided by Prof. Andrews' results, consider
what would probably be the successive changes in a condensing nebulous
spheroid.

Prof. Andrews has shown that for each kind of gaseous matter there is a
temperature above which no amount of pressure can cause liquefaction.
The remark, made _a priori_ in the above extract, "that if, under
increasing pressure, a gas retains all the heat evolved, its
resisting force is _absolutely unlimited_", harmonizes with the
inductively-reached result that if the temperature is not lowered to its
"critical point" a gas does not liquify, however great the force
applied.