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But as the nucleus faded towards the end of June, they came out strongly, and were more and more clearly seen, both at Greenwich and at Princeton, to agree, not with the spectrum of hydro-carbons glowing in a vacuum tube, but with that of the same substances burning in a Bunsen flame. It need not, however, be inferred that cometary materials are really in a state of combustion.
A large proportion of smoke consists of particles of pure unconsumed carbon, and this is accompanied in its passage up our chimneys by sulphurous acid, begotten by the sulphur which is contained in the coal to the amount of about eight pounds in every thousand; by sulphuretted hydrogen, by hydro-carbons, and by vapours of various kinds of oils, small quantities of ammonia, and other bodies not by any means contributing to a healthy condition of the atmosphere.
Marsh-gas is the first of the series of hydro-carbons known chemically as the paraffins, and is an extremely light substance, being little more than half the weight of an equal bulk of air.
Hydro-carbons had been shown by the spectroscope to be present in comets, and were fitted by their specific weight, as compared with that of hydrogen, to form tails of the second type; while the atoms of iron were just heavy enough to compose those of the third, and, from the plentifulness of their presence in meteorites, might be presumed to enter, in no inconsiderable proportion, into the mass of comets.
The oleaginous group comprises all oily matters, which are even purer hydro-carbons than the first-mentioned class. The third, or albuminous group, includes all substances closely allied to albumen, and hence containing a large proportion of nitrogen in addition to the other three elements.
These he found to be hydrogen for the first, hydro-carbons for the second, and iron for the third.
For the increasing thermal power of the sun might, indeed, have ignited the sodium, but it could not have extinguished the hydro-carbons. Sir William Huggins succeeded in photographing the spectrum of comet Wells by an exposure of one hour and a quarter. The result was to confirm the novelty of its character.
In the first type he calculates the repulsive force at from twelve to fifteen times the force of gravity; in the second at from two to four times; and in the third at about one and a half times. The straight tails he ascribes to hydrogen because the hydrogen atom is the lightest known; the sword-shaped tails to hydro-carbons; and the stumpy tails to vaporized iron.
Their illumination by disruptive electric discharges was, however, a condition sine quâ 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.
It is, first, the miraculous plasmic power in the grain of seed, which brings forth after its kind; then the alchemy of sunlight which, in presence of the green colouring matter of the leaves, gathers hydrogen from the water and carbon from the gases in the air, and mingles them in the hydro-carbons of plant growth; and, finally, the wholly occult vital powers of the plant itself, stored up through ages, and flowing down from the primal sources of life.
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