In 1868 Sir William Huggins carried the matter one step further by showing that the spectrum of Winnecke's comet of that year agreed with that of olefiant gas rendered luminous by electricity; and the presence of the hydrocarbon spectrum has since been detected in a large number of comets. The first really brilliant comet to be analyzed by the spectroscope was Coggia's (1874), and it presented not only the three bright bands that had been already seen, but the whole range of five bands characteristic of the hydrocarbon spectrum. In certain cases, however--notably, that of Holmes's comet of 1892 and that of the great southern comet of 1901 (Plate XXV.)--the spectrum has not exhibited the usual bright band type, but has instead shown merely a continuous ribbon of colour. From these analyses certain facts emerge. First, that the gaseous surroundings of comets consist mainly of hydrogen and carbon, and that in all probability their luminosity is due, not to mere solar heat, but to the effect of some electric process acting upon them during their approach to the sun; and second, that, along with these indications of the presence of luminous hydrocarbon compounds, there is also evidence of the existence of solid particles, mainly in the nucleus, but also to some extent in the rest of the comet, which shine by reflected sunlight. It is further almost certain, from the observation by Elkin and Finlay of the beginning of the transit of Comet 1882 (iii.) across the sun's face, that this solid matter is not in any sense a solid mass. The comet referred to disappeared absolutely as soon as it began to pass the sun's edge. Had it been a solid mass or even a closely compacted collection of small bodies it would have appeared as a black spot upon the solar surface. The conclusion, then, is obvious that the solid matter must be very thinly and widely spread, while its individual particles may have any size from that of grains of sand up to that of the large meteoric bodies which sometimes reach our earth. Thus the state of the case as regards the constitution of comets is, roughly speaking, this: They consist of a nucleus of solid matter, held together, but with a very slack bond, by the power of gravitation. From this nucleus, as the comet approaches perihelion, the electric action of the sun, working in a manner at present unknown, drives off volumes of luminous gas, which form the tail; and in some comets the waves of this vapour have been actually seen rising slowly in successive pulses from the nucleus, and then being driven backwards much as the smoke of a steamer is driven. It has been found also by investigation of Comet Wells 1882 and the Great Comet of 1882 that in some at least of these bodies sodium and iron are present.