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IOWA ACADEMY OP SCIENCE Vo£. XXIV, 1917 
system was originally in an extremely heated gaseous spheroid 
extending far beyond the present orbit of Neptune. This 
spheroid contracted and rotated as a result of loss of heat. In 
time an equatorial ring of gaseous matter was left behind in the 
orbit now occupied by Neptune. After further shrinkage other 
rings were formed where the other planets now revolve. As 
these rings cooled they parted and collected into spheroids which 
gradually condensed into the planets. Most of them while still 
gaseous gave off secondary rings which evolved into satellites. 
In those cases where cooling progressed far enough the masses 
liquified and at length their surfaces hardened into rock. A 
modification of the theory suggested that owing to pressure 
solidification would begin at the center, while on the contrary 
other students urged that the temperature at the center would 
be too high for the original gas ever to liquify. 
Now it will be conceded that there are many features of 
the solar system which seem to harmonize beautifully with this 
theory. It is certainly true also that the earth’s interior is hot 
and that vast quantities of molten rock have been thrust forth 
from within. And it is also true that most of the oldest known 
rocks are igneous or derived from igneous rocks. But on the 
other hand there have developed, especially in recent years, a 
number of serious objections. 
(1) Lord Kelvin computed that the density of the nebula 
when it was expanded forty times beyond the orbit of the earth 
(Neptune’s orbit has a radius thirty times that of the earth) 
would be 1/570,000,000 that of common air. It is difficult to 
understand how such a diffuse body coul'd maintain such an ex- 
ceedingly high temperature as postulated, and why its substance 
would not have cooled to solid particles long before these could 
become aggregated. 
(2) It has been urged that definite rings might not be formed 
but that the equatorial matter would separate particle by particle. 
(3) Mathematical calculations show difficulties in the way of 
a ring forming into a spheroid so simply as the theory demands. 
The earth ring would have a cross section of about twenty-five 
miles and its center of gravity would be at the center of the sun. 
Such a ring of gas with its exceedingly low gravitative force and 
with the high temperatures necessary to keep all the earth sub- 
stances in gaseous form could not hold together by its own 
