must apparently have formed part of an aeriform mass in which they were
immersed at an earlier stage of their history.

PLATE II.

[Illustration: Great Comet.

Photographed, May 5, 1901, with the thirteen-inch Astrographic Refractor
of the Royal Observatory, Cape of Good Hope.]

In a few exceptional comets the usual carbon-bands have been missed. Two
such were observed by Sir William Huggins in 1866 and 1867
respectively.[1261] In each a green ray, approximating in position to
the fundamental nebular line, crossed an otherwise unbroken spectrum.
And Holmes's comet of 1892 displayed only a faint prismatic band devoid
of any characteristic feature.[1262] Now these three might well be set
down as partially effete bodies; but a brilliant comet, visible in
southern latitudes in April and May, 1901, so far resembled them in the
quality of its light as to give a spectrum mainly, if not purely,
continuous. This, accordingly, is no symptom of decay.

The earliest comet of first-class lustre to present itself for
spectroscopic examination was that discovered by Coggia at Marseilles,
April 17, 1874. Invisible to the naked eye till June, it blazed out in
July a splendid ornament of our northern skies, with a just perceptibly
curved tail, reaching more than half way from the horizon to the zenith,
and a nucleus surpassing in brilliancy the brightest stars in the Swan.
Bredikhine, Vogel, and Huggins[1263] were unanimous in pronouncing its
spectrum to be that of marsh or olefiant gas. Father Secchi, in the
clear sky of Rome, was able to push the identification even closer than
had heretofore been done. The _complete_ hydro-carbon spectrum consists
of five zones of variously coloured light. Three of these only--the
three central ones--had till then been obtained from comets; owing, it
was supposed, to their temperature not being high enough to develop the
others. The light of Coggia's comet, however, was found to contain all
five, traces of the violet band emerging June 4, of the red, July
2.[1264] Presumably, all five would show universally in cometary
spectra, were the dispersed rays strong enough to enable them to be
seen.

The gaseous surroundings of comets are, then, largely made up of a
compound of hydrogen with carbon. Other materials are also present; but
the hydro-carbon element is probably unfailing and predominant. Its
luminosity is, there is little doubt, an effect of electrical
excitement. Zoellner showed in 18