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density in his central parts."[1] "As for the future, . . . inhabitants of the earth can not continue to enjoy the light and heat essential to their life, for many million years longer, unless sources now unknown to us are prepared in the great store-house of creation." More detailed studies of the same subject were made in 1887, in a lecture "On the Sun's Heat," delivered before the Royal Institution of Great Britain.[2]

In this lecture Sir William Thomson refers to a very able paper, "On the Theoretical Temperature of the Sun," by J. Homer Lane,[3] of Washington, which establishes the apparently paradoxical statement that, within certain limitations, the more heat a gaseous body loses by radiation, the hotter it will become. This theorem was discussed in connection with the solar heat by Benjamin Peirce,[4] Simon Newcomb[5] and Sir Robert Ball.[6] Results similar to Lane's were reached in the years 1878-83 in a series of very exhaustive papers by A. Bitter.[7] A rival to the Helmholtz-Thomson theory of solar heat was advanced about 1882 by William Siemens,[8] who imagined the rotating sun to hurl, by centrifugal action at his equator, enormous quantities of gas into space, which returned to him again at the poles.

A refinement of the theory as presented by Helmholtz was introduced in 1899 by T. J. J. See,[9] wherein he abandoned the Helmholtzian hypothesis of a sun of homogeneous density and, using Lane's law, investigated minutely the more complex case of central condensation. Thereby the probable solar age was extended from about 18 to about 32 million years.

Returning to the problem of the age of the earth, considered independently of the sun, we find William Thomson the great moving spirit. He approached the subject from more than one point of view. One argument for limitation of the earth's age was based on the consideration of underground heat.[10] "The heat which we know by

  1. Op. cit., p. 375.
  2. Op. cit., pp. 376-429.
  3. American Journal of Science, 2d S., Vol. 50, 1870, pp. 57-74.
  4. Proceed. Am. Acad., XV., p. 201.
  5. "Popular Astronomy," 1st ed., p. 508; "The Stars," New York, 1901, p. 210.
  6. "Story of the Heavens," London, 1893, p. 497.
  7. Wiedemann's Annalen, V., p. 405; X., p. 13; XI., p. 978; XII., p. 445; XIII, p. 360; XIV., p. 16; XVI., p. 166; XVII., p. 322; XVIII, p. 488; XX., pp. 137, 897. See Rosenberger, "Geschichte der Physik," Vol. III., 1887, pp. 689, 690.
  8. "Ueber die Erhaltung der Sonnenenergie," Uebersetzt von C. E. Worms, Berlin, 1885; see Rosenberger, "Geschichte der Physik," Vol. III., p. 687.
  9. Science, N. S., Vol. IX., 1899, pp. 737-740.
  10. W. Thomson, "The Doctrine of Uniformity in Geology Briefly Refuted," read in 1865 before the Royal Society of Edinburgh. See Smithsonian Report, 1897, p. 343.