WCP4986

[1]¹ [p. 440]

Frith Hill, Godalming,

12^th December 1885.

Dr. James Croll, F.R.S.

Dear Mr. Croll,—I have been reading, with very great interest and pleasure, the two last chapters of your book on the origin of sun's heat and of nebulae. Your [2] [p. 441] theory of the light and heat of all suns, stars, and nebulae being due to arrested motion seems to me most simple, suggestive, and probable, and supported by a mass of weighty fact and argument. It seems the complement of the molecular theory of gases, the molecules in this case being the vast molar masses of the stellar universe.

There is, however, a point connected with your discussion of the age of the sun's heat and of geological time that does not seem to me so clear. You say that the sun can only have been giving heat at the present rate for 20 million years if heat is due solely to gravitation, and that geology requires far more than 20 million years. Your arrested motion gives any possible amount of heat to begin with, and thus you get a longer period of emission.

But this seems to me rather beside the point. Whatever heat the sun originally had, it had cooled down to the present temperature by radiation, losing heat less and less rapidly as it became cooler. In doing so, it must have passed through the point at which life, such as we find in the earlier Palaeozoic rocks, first became possible, say a temperature such as would have heated the ocean to 120º or 150º F. Would not the time from that epoch to now be a fixed period, whatever the primitive heat of the sun? If so, this period is what we want to arrive at, not the whole period during which the sun has been giving out heat, which has no bearing on the problem. Thus let H be the heat of the sun at starting, H — N the heat now. It has cooled according to some definite law.

H.. H — a.. H — 3.. H — c...... H — N

If H — c represents the temperature at which life became possible on earth, then the time H — c... to H — N is what we want, and whether the time between H and H — c was 5 million or 5000 million years makes no difference whatever. Another limiting factor is the time required for the earth to cool from the highest possible life temperature to present temperature. This [[3] [p. 442] you allude to at end of Chapter xviii. as not calculable within sufficiently close limits, but you do not, I think, refer to the fact that the time of the sun's cooling, from the limits of terrestrial life temperature to present temperature, is not affected by the total store of its original heat. This point seems to me to want further elucidation.—Yours very faithfully,

Alfred R. Wallace.