202 Views on Frenehular Conditions. — A. Witichell. 
which favor the theory are well set forth in Dr. Croll's recent 
work on "Stellar Evolution." The motion of the colliding 
masses is not supposed to be entirely due to the action of 
gravity — as according to the more recent conception of Sir 
William Thomson.'* "If the masses were created, they may 
as likely have been created in motion as at rest; if they were 
eternal, they may as likely have been eternally in motion as 
eternally at rest. Eternal motion is just as warrantable an 
assumption as eternal matter." "A mass equal to that of the 
sun, moving with a velocity ^i 476 miles per second, would 
possess, in virtue of that motion, energy sufficient, if converted 
into heat, to maintain the present rate of the sun's radiation 
for 50,000,000 years." 
"The collision of two bodies each half the mass of the sun 
would result immediately in a chaos of fragments. The enor- 
mous heat generated would further shatter and disperse the 
fragments. Much of the matter would be transformed suddenly 
into the gaseous condition. In the course of time, the whole 
would assume the gaseous condition, and we should then have 
a perfect nebula'^ — intensely hot, but not very luminous. As 
the temperature diminished, the nebulous mass would begin to 
condense, and ultimately, accordingto the well known nebular 
hypothesis,pass through all the different phases of rings, planets 
and satellites, into our Solar system as it now exists." 
Here then, are two distinct conceptions of prenebular his- 
tory : — First, The Meteoric Theory, as reasoned out, it is rep- 
resented, by professor Tait, and independently by the present 
writer, and also, with convincing and admirable fullness,by pro- 
fessor Lockyer; Second, The Impact Theory, of which Dr. 
Croll's form postulates initial motion and Sir William Thom- 
son's assumes initial rest. Under the first theory, the nebu- 
lar mass undergoes a prolonged aggregation, heat resulting 
from the impact of descending contributions. Under the im- 
pact theory, dark bodies exist in immensity-" whose move- 
^'^Naiure, 27 Jan. 1887; Proc. Roy. Institution, vol. xii. 
^^ Whether "a perfect nebula" is entirely gaseous is the question at 
present in doubt. 
-'•'Lambert conceived the existence of dark bodies as centres of the 
great cosmic systems. "Kosmologische Briefe ueber die Einrichtung 
des Weltbaus," Augsburg, 1761. Nevvcomb affirms that "not the 
slightest evidence favoring the existence of these opaque centres has- 
ever been found." 
