WCP4987

[1]¹[p. 468]

Parkstone, Dorset,

25^th August 1889.

James Croll, Esq.

My dear Sir,—I have now been able to read your very interesting Stellar Evolution. I think your general idea of the stellar universe a very probable and suggestive one, as it accounts for the proper motion of the stars and other phenomena, which a general nebulous theory will not do. But I do not think that this theory in any way gets over the difficulty of the amount of solar heat and the duration of geological time. You appear to have overlooked the fact that the initial heat of the solar nebula has no hearing on the question at all, but only the heat at the time when it (the nebula) had contracted to the diameter of the earth's orbit and had thrown off the earth. Now, the heat at that epoch is a fixed quantity, not affected by the initial heat of the nebulae. It matters not, therefore, whether the original heat of the solar system was sufficient to expand the solar nebula to the diameter of the orbit of Neptune, or to ten times that diameter; when it had cooled down to the dimensions of the earth's orbit, it would have reached the same temperature and have contained the same store of heat. What we require is not a greater original heat, but some agency by which the sun continued to generate heat, if not so fast as it gives it out, yet sufficiently to delay the cooling almost indefinitely. Two such causes are [2] [p. 469] conceivable and have been suggested,—one derived from the constant rain of meteorites on the sun from the stellar space through which it passes during its proper motion through the stellar universe; the other the continued absorption of fresh gaseous matter from the ethereal spaces it is passing through—as suggested by Mr. Matthew Williams in his Fuel of the Sun. This objection seems to me so obvious that I cannot understand how it is you have not even noticed it. For your purpose it was necessary to show that, after the earth had been thrown off, and its surface had cooled sufficiently to allow of the condensation of water and the formation of sea and land, the sun could possibly contain many times more heat than was required to keep it at the diameter it had then attained. You must show, in fact, that it is possible to have two suns, identical in constitution, in mass, and in dimension, yet one containing many times as much heat as the other, because it had had a higher initial temperature. That seems to me to be physically impossible. Is it not so?—Yours very faithfully,

Alfred R. Wallace.