WCP4988

[1]¹ [p. 470]

Parkstone, Dorset,

12^th September 1889.

Dear Mr. Croll,—You seem to assume that in the nebular hypothesis the primitive nebula is assumed to be cold, and its condensation to be due solely to gravity. But did anybody ever hold this theory? I find in the article "Solar System" in the English Cyclopaedia written by Professor D. Morgan, the theory is stated to be that "condensation takes place arising from loss of heat," and again he refers to the necessary consequences "of the cooling of a nebulous atmosphere." And, surely, any other conception is absurd and physically inconceivable. Can anybody suppose the whole physical constitution of the universe to be so different from what it is now, that iron and all the other metals and solid elements could exist as cold vapours. The point of liquefaction of each metal, as well as the points of vaporisation of every solid element, are physical constants, and cannot be conceived to have been totally different from what they are now in any rational theory of the universe.

[2] [p. 471] I cannot therefore see that the theory of a "cold nebula," not leading to heat enough to produce the sun or the solar nebula by gravitation alone, has any bearing on the problem. The sun, at a certain epoch, say that of the Laurentian epoch, had condensed to a certain diameter, and it has since condensed to the diameter it now possesses. Each of these diameters, assuming the mass to be a fixed quantity, implies a definite amount of heat, which amount would not be affected by the fact of the original nebula having been hotter, and therefore larger, or cooler, and therefore smaller, since it must anyhow have been once at least as large as to fill the orbit of Neptune. This objection your letter does not touch.

I return Dr. Huggins' paper, which I had seen in Nature.—Believe me, yours faithfully,

Alfred R. Wallace.