e comet and the meteors could scarcely be more
clearly proved; while the vast dimensions of the stream into which the
latter are found to be diffused cannot but excite astonishment not
unmixed with perplexity.

The first successful application of the spectroscope to comets was by
Donati in 1864.[1255] A comet discovered by Tempel, July 4, brightened
until it appeared like a star somewhat below the second magnitude, with
a feeble tail 30 deg. in length. It was remarkable as having, on August 7,
almost totally eclipsed a small star--a very rare occurrence.[1256] On
August 5 Donati admitted its light through his train of prisms, and
found it, thus analysed, to consist of three bright bands--yellow,
green, and blue--separated by wider dark intervals. This implied a good
deal. Comets had previously been considered, as we have seen, to shine
mainly, if not wholly, by reflected sunlight. They were now perceived to
be self-luminous, and to be formed, to a large extent, of glowing gas.
The next step was to determine what _kind_ of gas it was that was thus
glowing in them; and this was taken by Sir William Huggins in
1868.[1257]

A comet of subordinate brilliancy, known as comet 1868 ii., or sometimes
as Winnecke's, was the subject of his experiment. On comparing its
spectrum with that of an olefiant-gas "vacuum tube" rendered luminous by
electricity, he found the agreement exact. It has since been abundantly
confirmed. All the eighteen comets tested by light analysis, between
1868 and 1880, showed the typical hydro-carbon spectrum[1258] common to
the whole group of those compounds, but probably due immediately to the
presence of acetylene. Some minor deviations from the laboratory
pattern, in the shifting of the maxima of light from the edge towards
the middle of the yellow and blue bands, have been experimentally
reproduced by Vogel and Hasselberg in tubes containing a mixture of
carbonic oxide with olefiant gas.[1259] Their illumination by disruptive
electric discharges was, however, a condition _sine qua non_ for the
exhibition of the cometary type of spectrum. When a continuous current
was employed, the carbonic oxide bands asserted themselves to the
exclusion of the hydro-carbons. The distinction has great significance
as regards the nature of comets. Of particular interest in this
connection is the circumstance that carbonic oxide is one of the gases
evolved by meteoric stones and irons under stress of heat.[1260] For it