"
And whilst facing with brave and steady mind the great mysteries of
earth and sky, of life and what lies beyond it, he himself loved to
quote:
"Fear not thou the hidden purpose
Of that Power which alone is great,
Nor the myriad world His shadow,
Nor the silent Opener of the Gate."
Among the scientific friends to whom he appealed for help when writing
his astronomical books was Prof. (now Sir) W.F. Barrett.
* * * * *
TO PROF. BARRETT
_Parkstone, Dorset. February 12, 1901._
My dear Barrett,--I shall be much obliged if you will give me your
opinion on a problem in physics that I cannot find answered in any book.
It relates to the old Nebular Hypothesis, and is this:
It is assumed that the matter of the solar system was once wholly
gaseous, and extended as a roughly globular or lenticular mass beyond
the orbit of Neptune. Sir Robert Ball stated in a lecture here that even
when the solar nebula had shrunk to the size of the earth's orbit it
must have been (I think he said) hundreds of times rarer than the
residual gas in one of Crookes's high vacuum tubes. Yet, by hypothesis,
it was hot enough, even in its outer portions, to retain all the solid
elements in the gaseous state.
Now, admitting this to be _possible_ at any given epoch, my difficulty
is this: how long could the outer parts of this nebula exist, exposed to
the zero temperature of surrounding space, without losing the gaseous
state and aggregating into minute solid particles--into meteoric dust,
in fact?
Could it exist an hour? a day? a year? a century? Yet the process of
condensation from the Neptunian era to that of Saturn or Jupiter must
surely have occupied millions of centuries.
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