skip to content

Darwin Correspondence Project

From J. W. Strutt   [after 20 May 1871?]1

Much of the Author’s theory seems to depend on the assumed transparency of flame. See p 77.2 But the theory of exchanges (supported by an immense weight of evidence) so far from proving this as the Author states, really asserts just the contrary.3 No thickness of flame at a given tempre. can radiate more than a single lamp black surface at the same tempre. wd. do. The view in the book is entirely inconsistent with the facts of spectrum analysis.4 This being so, the argument from dissociation wd. tend to shew that the Sun’s heat cannot possibly be due to burning hydrogen, for in order that the total heat radiated sd. be as great as we find it, the solar temperature must be so high that hydrogen would not burn.

Many of the minor points brought forward seem to be established. Thomson5 I believe agrees that there is no particular limit to the atmosphere, tho the idea that the interior of the Sun could be cool below the flaming atmosphere is shewn to be in the highest degree improbable.6

I am not competent to judge with decision on the astronomical parts, but the Author does not seem to be acquainted with Thomson’s papers, nor to understand the bearing on the questions he discusses of the laws of Thermodynamics wh. are almost universally admitted.7


The date is conjectured from the relationship between this letter and the letter from A. R. Wallace, 14 May 1871, and from Strutt’s visit to Down on 20 May 1871 (see n. 2, below).
Alfred Russel Wallace recommended the book The fuel of the sun by William Mattieu Williams (Williams 1870) in his letter of 14 May 1871. CD’s copy is in the Rare Books Department–CUL. When Strutt visited Down on 20 May 1871 (Emma Darwin’s diary (DAR 242)), CD probably asked for his opinion on it.
In Williams 1870, pp. 77–8, Williams argued that the theory of exchanges supported the assumption that all flames are diathermic to radiations of similar quality and intensity and inferred that the diathermacy of flame resulted in ‘its power of cumulative thermal radiation’. The theory of exchanges, developed within the caloric theory of heat, postulated a system of dynamic equilibrium in which all bodies radiated and absorbed heat. Equilibrium was reached when the amount of heat radiated from a body equalled that which it absorbed.
Williams believed that an atmosphere similar to that on earth but in a more attenuated state pervaded all of space (Williams 1870, pp. 25–9). Developments in spectroscopy, such as the explanation of Fraunhofer lines in 1859, allowed researchers to begin to analyse the chemical composition of stars.
William Thomson.
See Williams 1870, pp. 78–9.
In Williams 1870, pp. 58–9, Williams argued that heat radiated into space was reabsorbed by the sun as it travelled through space. Such a process would violate the second law of thermodynamics. Thomson’s expression of the law stated that heat could not be conveyed from one body to another at a higher temperature (Thomson 1851, p. 266).


Criticises a book [W. M. Williams, The fuel of the sun (1870)] whose author does not understand thermodynamics, spectrum analysis, and W. Thomson’s papers.

Letter details

Letter no.
John William Strutt, 3d Baron Rayleigh
Charles Robert Darwin
Sent from
Source of text
DAR 176: 26
Physical description
L 2pp inc

Please cite as

Darwin Correspondence Project, “Letter no. 7787,” accessed on 17 July 2019,

Also published in The Correspondence of Charles Darwin, vol. 19