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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. What kept the almost
infinitely rare metallic gases in the gaseous state all this time? Is
such a condition of things physically possible?

I cannot myself imagine any such condition of things as the supposed
primitive solar nebula as possibly coming into existence under any
conceivably antecedent conditions, but, granted that it did come into
existence, it seems to me that the gaseous state must almost instantly
begin changing into the solid state. Hence I adopt the meteoric theory
instead of the nebular; since all the evidence is in favour of solid
matter being abundant all through known space, while there is no
evidence of metallic gases existing in space, except as the result of
collisions of huge masses of matter. Is my difficulty a mare's
nest?--Yours very truly,

ALFRED R. WALLACE.

* * * * *

TO Mrs. Fisher

_Broadstone, Wimborne. February 28, 1905._

Dear Mrs. Fisher,--Thanks for your letter. Am sorry I have not converted
you, but perhaps it will come yet! I will only make one remark as to
your conclusion.

I have not attempted to prove a negative! That is not necessary. What I
claim to have