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. Zöllner showed in 1872[1265] that, owing to evaporation and other changes produced by rapid approach to the sun, electrical processes of considerable intensity must take place in comets; and that their original light is immediately connected with these, and depends upon solar radiation, rather through its direct or indirect electrifying effects, than through its more obvious thermal power, may be considered a truth permanently acquired to science.[1266] They are not, it thus seems, bodies incandescent through heat, but glowing by electricity; and this is compatible, under certain circumstances, with a relatively low temperature.
The gaseous spectrum of comets is accompanied, in varying degrees, by a continuous spectrum. This is usually derived most strongly from the nucleus, but extends, more or less, to the nebulous appendages. In part, it is certainly due to reflected sunlight; in part, most likely, to the ignition of minute solid particles.
FOOTNOTES:
[1188] Month. Not., vol. xix., p. 27.
[1189] Mém. de l'Ac. Imp., t. ii., 1859, p. 46.
[1190] Harvard Annals, vol. iii., p. 368.
[1191] Ibid., p. 371.
[1192] Month. Not., vol. xxii., p. 306.
[1193] Stothard in Ibid., vol. xxi., p. 243.