tellation.

Clusters of stars are usually globular in form, though some are
irregular in outline. The latter are generally rich in stars, with a
less condensation of stars towards the centre. Sir Wm. Herschel
considered the irregular clusters as being in a less advanced stage of
condensation, as he was of the opinion that all groups ultimately tended
to clusters which were globular in form. Before dealing with the
different kinds of true nebulae we will now consider the question as to
"What are Nebulae?"

ART. 120. _What are Nebulae?_--The question which presents itself to the
mind of all astronomers when they have viewed the wondrous nebulae that
exist in far-off space is, "What are Nebulae?" This question is so
closely identified with the question as to "What is Matter?" that the
solution of the one will give us the key to the solution of the other.
It is now generally admitted, that nebulae are composed of a glowing
mass of gaseous matter, that gaseous matter being partly composed of the
gas Hydrogen. Dr. Huggins in 1864 first made the discovery of the
existence of Hydrogen in certain nebulae by means of the spectroscope,
which distinctly revealed certain lines that proved the existence of
Hydrogen in the nebulae.

In the spectra of some of the nebulae, that of 31 Andromeda, for
example, there are no dark lines shown, but only a continuous band of
bright light, which would seem to indicate that there was no glowing
gaseous matter in that nebula at all. But accepting the fact that the
nebulae are composed of glowing gaseous matter, the problem confronts us
as to where this gaseous matter comes from.

If, as spectrum analysis seems to teach us, there are nebulae in various
stages of formation, there must be a period in their history of
development when they had an origin. What, then, is the origin of a
nebula, and what the physical explanation of that origin? From optical
phenomena we learn that all space is not empty, but filled with the
Aether which is universal (Art. 42). What is the relation, then, of this
glowing nebulous matter to this universal Aether? If it be suggested
that there is no relation, then we are in the unphilosophical position
of having to admit, either that the nebulous matter of which the nebulae
are composed never had any origin, or that it had its origin in some
unknown and still undiscovered medium which exists in space. But both of
these hypotheses are unphilosophical, as the former is