the rarity of nebular gases. The residual gases in a vacuum tube are
dense by comparison. A cubic inch of air at ordinary pressure would
contain more matter than is contained in millions of cubic inches of the
gases of nebulae. The light of even the faintest stars does not seem to
be dimmed by passing through a gaseous nebula, although we cannot be
sure on this point. The most remarkable physical fact about these gases
is that they are luminous. Whence they derive their luminosity we do not
know. It hardly seems possible to believe that extremely thin gases
exposed to the terrific cold of space can be so hot as to be luminous
and can retain their heat and their luminosity indefinitely. A cold
luminosity due to electrification, like that of the aurora borealis,
would seem to fit the case better.

Now the nebular theory is that out of great "fire-mists," such as we
have described, stars are born. We do not know whether gravitation is
the only or even the main force at work in a nebula, but it is supposed
that under the action of gravity the far-flung "fire-mists" would begin
to condense round centres of greatest density, heat being evolved in the
process. Of course the condensation would be enormously slow, although
the sudden irruption of a swarm of meteors or some solid body might
hasten matters greatly by providing large, ready-made centres of
condensation.

Spiral Nebulae

It is then supposed that the contracting mass of gas would begin to
rotate and to throw off gigantic streamers, which would in their turn
form centres of condensation. The whole structure would thus form a
spiral, having a dense region at its centre and knots or lumps of
condensed matter along its spiral arms. Besides the formless gaseous
nebulae there are hundreds of thousands of "spiral" nebulae such as we
have just mentioned in the heavens. They are at all stages of
development, and they are visible to us at all angles--that is to say,
some of them face directly towards us, others are edge on, and some are
in intermediate positions. It appears, therefore, that we have here a
striking confirmation of the nebular hypothesis. But we must not go so
fast. There is much controversy as to the nature of these spiral nebulae.
Some eminent astronomers think they are other stellar universes,
comparable in size with our own. In any case they are vast structures,
and if they represent stars in process of condensation, they must be
giving birth to huge ag