THE ORIGIN AND DEVELOPMENT OF THE EARTH 107 
class is characterized by bright spectral lines: which indicate 
a structure somewhat akin to gaseous, although it is not cer- 
tain that this is the actual condition. Due to the fact that 
these nebulae show the presence of some elements not known 
to exist in any part of the solar system, and since there is 
absolutely no indication of metals in their constitution, they 
have been ruled out. It is, of course, conceivable that the 
elements composing these nebulae might in the course of time 
become elements such as we know, but is mere supposition, 
and will not suffice as a ground for basing a hypothesis. 
The other class of nebulae give what are called “continous 
line” spectra, which is commonly interpreted to mean that the 
materials composing them are in either liquid or solid state. 
It is also almost certain that these nebular materials are in 
very finely divided particles, for in spite of the immense size 
of the nebulae they are known to intercept very little light and 
possess but slight gravitativespower. Their spectra show, it 
seems, the presence of the same elements that compose the 
solar system, and their number is at least ten times as great 
as that of the nebulae of the “bright line’ type. 
The dominant type of these nebulae is the spiral, as was 
determined by the great astronomer Keeler, for years the 
director of the Lick Observatory. The distinguishing 
characteristic of the spiral nebulae is a central mass or ball 
with two arms which arise from opposite sides of the central 
mass and curve concentrically away from it. In the outer 
regions of these arms they commonly branch, but throughout 
all of the spiral nebulae the two dominant arms may be dis- 
tinguished. In these nebular arms there are also considerable 
knots between which the nebulous matter is irregularly dis- 
tributed. It is clear, from oblique views of the nebulae, such 
as that of Andromeda pictured in the accompanying plate, 
that the spirals are roughly disk-like, a shape which corres- 
ponds with that of the solar system. t 
The results of their study of the various characteristics of 
the spiral nebulae convinced Chamberlin and his associates 
that here, if anywhere among the astronomical bodies, they 
might successfully look for the traces of the earliest history 
of the solar system. While nothing is as yet known of the 
motions of the parts of these spirals, their shape seems to 
