2o6 The American Geologist. ^v"""- ^^^^ 
An inquiry into the possible modes by which the planetesimal 
condition might arise revealed several possible methods. Such 
condition might arise from a nebula that was originally gaseous. 
If. for example, it be supposed that the parent nebula was a gaseous 
spheroid, and that it detached material from its equatorial belt 
molecule by molecule, rather than by rings, as postulated by La- 
place, these molecules would probably become planetesimals in- 
stead of members of a true gaseous body. * * * There is rea- 
son to believe that this method would really be almost the only 
systematic one by which a gaseous spheroid of the Laplacian type 
would detach material from its equatorial belt. * * * 
* * * To develop the hypothesis as definitely and concretely 
as possible, I have further chosen a special case from among those 
that might possibly arise, the case in which the nebula is supposed 
to have arisen from the dispersion of a sun as a result of close 
approach to another large body. The case does not involve the 
origin of a star nor even the primary origin of the solar system, 
but rather its rejuvenation and the origin of a new familj' of 
planets. * * * This particular sub-hypothesis was selected for 
first development (1) because it postulates as simple an event as it 
seems possible to assign as the source of so great results, (2) be- 
cause that event seems very likely to have happened, (3) because 
the form of the nebula supposed to arise in this way is the most 
common form known, the spiral, and (4) because spectroscopic 
observations seem at present to support the constitution assigned 
this class of nebulae. * * * 
* * * The continuous spectrum is interpreted to mean that 
their chief luminous material is in a liquid or solid state. * * * 
As the liquid condition is liniited to a rather narrow range of 
temperature, and as this range is very different for different 
material, it is improbable that any large portion of a nebula is in 
this state, and the whole may be conveniently treated as though 
it were formed of solid matter, but matter in a fine divided con- 
dition. This last qualification seems necessary, for the volume 
of these nebulae is often very great, and yet they appear to inter- 
cept but little light and give no signs of great attractive power. 
The prevailing form of these nebulae is the spiral as deter- 
mined by the late Professor Keeler, and this form particularly 
characterizes the smaller nebulae recently brought to knowledge 
by improved instruments and manipulative skill. These newly 
discovered nebulae are estimated to number at least ten times the 
whole number previously known. From the superior number of 
spiral nebulae it is a safe inference that their peculiar forms repre- 
sent some prevalent process in celestial dynamics This is in 
itself a reason why research should turn to them, by preference, 
for the origin of the present solar system. * * * 
A notable and seemingly very significant feature of these 
