104 THE AMERICAN MIDLAND NATURALIST 
perfect vacuum that man has been able to produce. How such 
a nebula, too thin to be perceived by any of our instruments, 
could have held itself together, and could have retained its 
heat for any length of time is impossible to understand. Like- 
wise it is impossible that such a nebula should have given off 
a single ring, even at its earliest stages, or how such a ring, 
had it been formed, could have condensed into a sphere that 
could in time become a planet. As well ask the ring of smoke 
blown from your cigar or pipe to become a ball. 
There are also movements of certain of the planets and 
their satellites that argue strongly against the Laplacian 
idea. If satellites evolved from rings that come from rotating 
planets, they should revolve around those planets in the same 
directation and with the same speed that the planets them- 
selves turn upon their axes. Now the inner satellite of Mars 
revolves about the planet three times while Mars turns on its 
axis once, and the ninth satellite of Saturn has been shown to 
move in a direction opposite to the one in which the planet 
itself turns. Under the Laplacian hypothesis these things 
could not be, yet they unquestionably have been observed. 
There is, however, one other line of argument which would 
dispose of the ‘‘nebular” hypothesis even though there were no 
other points against it. The moment, or amount, of momen- 
tum of any freely rotating system such as that to which our 
earth belongs must forever remain constant; that is a well- 
established principle of physics. In any ancestor of our solar 
system the moment of momentum must have equaled that of 
the present system, for the matter composing the one com- 
poses the other. But we find that such a nebula as the one 
postulated by Laplace could not have thrown off a ring until 
it had shrunk far within the orbit of the innermost planet. 
In order for this nebula to have produced the supposed ring 
from which Neptune was to descend it must have possessed at 
at least 200 times the momentum that is in the solar system 
today. And yet the moment of momentum of any freely ro- 
tating system must forever remain constant. 
Or let us consider matters from another angle. If the 
Laplacian hypothesis were correct, the amount of momentum 
which a planetary ring could possess should be directly pro- 
portional to the amount of material in that ring; the greater 
