faint spectrum upon which these bands are projected existed, or at least was visible, only in some cases. Comets whose nuclei are very faint, like that of Encke's comet, or not sufficiently luminous (comet 1873, IV.), have failed to give a continuous spectrum. We may consider, therefore, that the bright bands are not produced by the light of cometary atmospheres or comæ. From his first observations Mr. Huggins came to an opposite conclusion, but this was doubtless owing to the impossibility of comparing the results then obtained with those afforded by the comets which have been analysed since.

We may thus regard the comets with nuclei which have been analysed by the spectroscope as constituted as follows:-

In the centre of the nebulosity a nucleus giving a continuous spectrum. Does this necessarily imply a liquid or solid incandescent matter? We might answer in the affirmative if the continuity of the spectrum could be regarded as complete; but it is so faint that it is difficult to say with certainty whether the light with which it shines really belongs to the incandescent matter of which it is composed, or if it is light reflected from the sun. It is not improbable that this light is of both kinds, especially when the comet is drawing near the sun and is subjected to a continually increasing temperature. The observations of polarisation by reflection prove that in any case a part of the light is reflected from the sun.

As regards the light of the atmospheres and tails, the spectrum of bright bands denotes alike the gaseous and the incandescent state of the matter of which they are composed. The identity in this respect of the tail and coma of Coggia's comet clearly shows that it is the matter of the atmosphere which, under the influence of a repulsive action, helps to form the cometary appendage opposite the sun. As, on the other hand, the phenomena of sectors emanating from the nucleus prove that the atmospheric envelopes are formed at the expense

of the nucleus, it is very difficult to admit the incandescent state of the cometary atmosphere and tail without admitting that the nucleus, the seat of their continual formation, is likewise in an incandescent state. It is, then, probable that the nucleus, at all events in the vicinity of the perihelion, emits, besides light reflected from the sun, direct light that has emanated from its own substance.

In a chemical point of view the comets-few in number, it is true—which have as yet been subjected to examination are of very simple constitution. They consist of simple carbon, or of a compound of carbon and hydrogen, according to the comparisons made by Mr. Huggins; carbonic oxide or carbonic acid, according to the researches of Father Secchi. The Italian astronomer was, therefore, justified in saying: 'It is very remarkable that all the comets observed up to the present time have the bands of carbon,'

SECTION VII.

THE COMET OF 1874, OR COGGIA'S COMET.

Of the five comets of 1874 the third, or comet of Coggia, was alone visible to the naked eye-Telescopic aspect and spectrum of the comet during the early part of its apparition, according to Messrs. Wolf and Rayet-Observations of Secchi, Bredichin, Tacchini, and Wright; polarisation of the light of the nucleus and tailTransformations in the head of the comet between the 10th of June and the 14th of July, according to Messrs. Rayet and Wolf.

THE Comets, and not the comet, of 1874 should form the title, strictly speaking, of the present section of our work. Indeed, at the time of adding these lines to this chapterthat is to say, in the last few days of the month of August of this year [1874]-five new comets have been discovered and observed. But one only, the third in order of date, has attracted the attention of the public, for the simple reason that it alone became bright enough during the time of its apparition to be visible to the naked eye. The other four continued to remain telescopic comets, accessible only to professional astronomers. Although its visibility in Europe was of brief duration, the comet of 1874, III., or comet of Coggia, presented in its physical aspect, and in the changes of form in its head and tail, sufficiently curious phenomena to merit special mention and some detailed description.

At the Observatory of Marseilles, on the night of April 17, the new comet was discovered by an astronomer of that establishment, M. Coggia, already known in the scien