548 Proceedings of Royal Society of Edinburgh. [sess. 
preferred to the Kantian attempt to explain this quality of solar 
matter from an untenable mathematical and physical point of view. 
If the Laplaceian hypothesis would otherwise satisfactorily account 
for the development of our system, we might well grant his assump- 
tion that rotation was due to an external impulse beyond the 
grasp of our intelligence. But a recent criticism of the hypothesis 
has shown that our minds cannot readily accept all the con- 
clusions drawn in this great poem of cosmic evolution. In a paper 
contributed to the Astrophysical Journal , vol. xi., Mr Moulton, 
a mathematical astronomer of high repute, attacks the hypothesis 
from various mathematical and physical points of view. His 
negative conclusions appear in many respects sufficiently sound 
and vigorous to convey the impression that the evolution of 
our system must have differed very largely from the ideal picture 
of Laplace. Since a brief review of Mr Moulton’s arguments 
seems necessary in order to understand more clearly the bearing 
of our own hypothesis upon solar evolution, I beg to quote 
a few passages from his work which may give an idea of the 
nature and extent of the difficulties encountered in the nebular 
hypothesis. 
(P. 104.) “ The methods of testing the theory will he divided 
into three categories : — (i.) Comparison of observed phenomena 
with those which result from the expressed or implied conditions 
maintained by the hypothesis ; (ii.) Answers to the question 
whether the supposed initial conditions could have developed into 
the existing system ; (iii.) Comparison of those properties of the 
supposed initial system with the one now existing, which are 
invariant under all changes resulting from the action of internal 
forces.” 
(P. 129.) “Under the methods of the first category certain 
phenomena are enumerated which contradict the hypothesis so 
flatly that candid minds must admit that its validity in the form 
considered is open to serious question. In less exact sciences 
such objections would overthrow a theory or lead to its reconstruc- 
tion. The objections are, that the planes of the planets’ orbits 
present considerable deviations, while four satellites revolve in 
planes making practically right angles with the average of the 
system ; that the distribution of mass in the planets is unaccount- 
