Cyanogen is an electrically similar compound of carbon with the addition of nitrogen. Of these elements it will be seen that nitrogen and hydrogen are found to exist also in the gaseous nebulæ, and with the probable addition there of oxygen; but in comets the quota of active oxygen must be sought for in the correlated planetary, and not in the cometic, atmospheres, as is the case with the sun. Of the presence of the vapor of carbon in comets Professor Ball says, "This is a very singular fact, when it is remembered that carbon is one of the substances essentially associated with life in the forms in which we know it." Professor Huggins says, "Since that time the light from some twenty comets has been examined by different observers. The general close agreement in all cases, notwithstanding some small divergencies, of the bright bands in the cometary light with those seen in the spectrum of hydrocarbons justifies us fully in ascribing the original light of these comets to matter which contains carbon in combination with hydrogen." We may learn something further of the constitution of comets, perhaps, by considering the chemical reactions which their spectra seem to indicate. The following extract is from a recent article on the manufacture of illuminating gas: "Ammonia contains 82.35 parts of nitrogen and 17.65 of hydrogen. It is not produced by a direct combination, for nitrogen can be caught and wedded only by a hot and skilful wooing. In the gas retort, at a temperature of 2200 degrees and in the presence of lime, soda, or potash, it will combine with carbon and form cyanogen, and then further combine with the alkali to form a cyanide. There is steam in the retort, and, as nearly as the gas chemists can make out, the nitrogen promptly divorces itself, gives up the carbon to the oxygen of the steam, and, taking the hydrogen to itself, becomes, for the time at least, a fixed, if volatile, substance, but ever ready to enter into new alliances." It will be remembered that in the comets examined by Professors Huggins and Draper the spectroscope revealed both cyanogen and the double line of sodium. The function of the sodium is readily understood, as by its presence it enables the nitrogen in the cometic atmosphere to combine with a part of the carbon of the gaseous hydrocarbons which constitute this atmosphere, and thus produce the cyanogen. But to effect this combination requires in the retort a temperature of 2200 degrees. If the combining temperature around the nucleus of a comet is the same, it will show that the temperature of this comet's nucleus must be very high, and, while many times less than that of the sun's photosphere, it still clearly illustrates the powerful character of the impact of the planetary electrical currents upon the comet, and its tremendous repulsion by the similarly electrified solar electrosphere. The second one of the above reactions, that from cyanogen to ammonia, is due to the steam or aqueous vapor in the retort.