pes, and the number is likely to be increased. A German
astronomer recently counted 1528 on one photographic plate. Many of
them, moreover, are so vast that they must contain the material for
making a great number of worlds. Examine a good photograph of the nebula
in Orion. Recollect that each one of the points of light that are
dotted over the expanse is a star of a million miles or more in diameter
(taking our sun as below the average), and that the great cloud that
sprawls across space is at least 10,000 billion miles away; how much
more no man knows. It is futile to attempt to calculate the extent of
that vast stretch of luminous gas. We can safely say that it is at least
a million times as large as the whole area of our solar system; but it
may run to trillions or quadrillions of miles.

Nearly a hundred other nebulae are known, by the spectroscope, to be
clouds of luminous gas. It does not follow that they are white-hot, and
that the nebula is correctly called a "fire-mist." Electrical and other
agencies may make gases luminous, and many astronomers think that the
nebulae are intensely cold. However, the majority of the nebulae that
have been examined are not gaseous, and have a very different structure
from the loose and diffused clouds of gas. They show two (possibly more,
but generally two) great spiral arms starting from the central part and
winding out into space. As they are flat or disk-shaped, we see this
structure plainly when they turn full face toward the earth, as does the
magnificent nebula in Canes Venatici. In it, and many others, we clearly
trace a condensed central mass, with two great arms, each apparently
having smaller centres of condensation, sprawling outward like the
broken spring of a watch. The same structure can be traced in the mighty
nebula in Andromeda, which is visible to the naked eye, and it is said
that more than half the nebulae in the heavens are spiral. Knowing that
they are masses of solid or liquid fire, we are tempted to see in them
gigantic Catherine-wheels, the fireworks of the gods. What is their
relation to the stars?

In the first place, their mere existence has provided a solid basis for
the nebular hypothesis, and their spiral form irresistibly suggests
that they are whirling round on their central axis and concentrating.
Further, we find in some of the gaseous nebulae (Orion) comparatively
void spaces occupied by stars, which seem to have absorbed the nebulous
matter