Address to the Royal Geological Society. 9 
course the reason of this discrepancy is that the estimates were 
made on different data, or, to use the more correct expression, 
assumptions. One assumption that is always made is, that the 
original nebula was cold when it began to fall together, and also 
that it had nothing but the potential energy of gravitation with 
which to accomplish its evolution ; of course some such assump- 
tions must be made before any calculation can be applied. 
But Dr. Croll well remarks, “It is quite conceivable that the 
nebulous mass may have been possessed of an original store of 
heat previous to condensation.” As he says, it may have been its 
heat that was the very cause of its condition of separation.* If 
we ask how the nebula came into its dissociated condition, we 
shall only be doing what the physicists themselves would do in 
any other physical question, that is, endeavouring to trace back- 
wards the steps of physical causation as far as possible. The 
moment of rotation possessed by the solar system, as a whole, 
shows that the original nebula did not start simply with the 
potential energy of the mutual gravitation of its parts. Its 
moment of rotation can only have been acquired through its 
external relations with other masses, or through the operation 
of forces acting from without. This is quantitatively quite 
insignificant in itself, comparatively speaking ; and if it were the 
full result of the action that produced it, it would be of very 
slight importance. But it is very improbable that it is no more than 
this: It is an indication that there has been an external action 
on the solar system nebula, the nature of which is unknown 
and this opens the door to vast possibilities as to the amount 
of thermal energy that may have been produced by that action, 
and been available for lengthening geological time. Dr. Croll 
has suggested that the solar system nebula may have been formed 
by the enormous amount of heat that would be produced by the 
collision of two sufficient cosmical masses. That such collisions 
have frequently occurred to cause the sudden blazing forth of new 
stars can hardly be doubted. The spectroscopic indications seem 
to show that it was something of this kind, on a small scale, that 
caused the sudden temporary increase of the star in Cygnus that 
has lately attracted so much attention. Professor Kirkwood has 
lately given reasons for believing that Sirius and his principal 
A * Climate and Time, p. 530, 
