ges, chemical processes,[649] or even through the mere reheating
of gases cooled by expansion.[650] All the appearances are against such
evasions of the difficulty presented by velocities stigmatised as
"fabulous" and "improbable," but which, there is the strongest reason to
believe, really exist.

On the 12th of December, 1878, Sir Norman Lockyer formally expounded
before the Royal Society his hypothesis of the compound nature of the
"chemical elements."[651] An hypothesis, it is true, over and over again
propounded from the simply terrestrial point of view. What was novel was
the supra-terrestrial evidence adduced in its support; and even this had
been, in a general and speculative way, anticipated by Professor F. W.
Clarke of Washington.[652] Lockyer had been led to his conclusion along
several converging lines of research. In a letter to M. Dumas, dated
December 3, 1873, he had sketched out the successive stages of
"celestial dissociation" which he conceived to be represented in the sun
and stars. The absence from the solar spectrum of metalloidal absorption
he explained by the separation, in the fierce solar furnace, of such
substances as oxygen, nitrogen, sulphur, and chlorine, into simpler
constituents possessing unknown spectra; while metals were at that time
still admitted to be capable of existing there in a state of integrity.
Three years later he shifted his position onward. He announced, as the
result of a comparative study of the Fraunhofer and electric-arc spectra
of calcium, that the "molecular grouping" of that metal, which at low
temperatures gives a spectrum with its chief line in the blue, is nearly
broken up in the sun into another or others with lines in the
violet.[653] This came to be regarded by him as "a truly typical
case."[654]

During four years (1875-78 inclusive) this diligent observer was engaged
in mapping a section of the more refrangible part of the solar spectrum
(wave-lengths 3,800-4,000) on a scale of magnitude such that, if
completed down to the infra-red, its length would have been about _half
a furlong_. The attendant laborious investigation, by the aid of
photography, of metallic spectra, seemed to indicate the existence of
what he called "basic lines." These held their ground persistently in
the spectra of two or more metals after all possible "impurities" had
been eliminated, and were therefore held to attest the presence of a
common substratum of matter in a simpler state of