of the conversion of nebulæ into stars." And not, it may be added, of stars into nebulæ. Of the irregular nebulæ, Professor Proctor says, "It may well chance, as long since suggested by Professor Clark, of Cincinnati, and as more cautiously hinted by Dr. Huggins, that in the varieties of constitution observed in the irregular nebulæ, and the evidence such varieties afford of progressive changes, we may find not merely direct evidence of the development of suns and sun-systems from the great masses of nebulous matter, but even what would be a far more important and impressive result,--actual evidence of the development of so-called elements from substances really elementary, or, at any rate, one stage nearer the elementary condition than are our hydrogen, nitrogen, oxygen, carbon, and so forth. The peculiarity of the spectral indications of the presence of nitrogen and hydrogen in the nebula, that only one line of nitrogen and two or three lines of hydrogen are discernible, instead of a complete spectrum of either element as seen under any known conditions, seems suggestive of what may be called a more elemental condition of hydrogen and nitrogen." Whether this be so, or whether these peculiarities are due to self-obscuration, or mutual reversal of the familiar lines due to the enormous disturbances of the nebular mass which must exist, it is certain that there is one terrestrial substance, at least, which acts invariably, in combination and chemical affinity, as a simple element in inorganic chemistry, but which is, in fact, compound,--to wit, the hypothetical radical ammonium, which is closely allied with the simple alkaline metals potassium and sodium, forming with them a single group; and yet, while the others have always remained as fixed, primitive elements, the hypothetical element ammonium alone is a composite substance consisting of hydrogen and nitrogen, two of the invariable gaseous constituents of all these nebulæ. In comets we find, vaguely expressed, an occasional strongly marked sodium line, and also the spectrum of carbon; in these gaseous nebulæ we find, as yet, no trace of carbon, and this element is so closely allied to hydrogen in its chemical affinities and reactions as to suggest that it may be the same element or some alloy of it, or in some allotropic form, as we find to be the case with other simple elements under special conditions. In organic chemistry--the chemistry of organic life--we find almost innumerable compound radicals which act as simple elements in combination, but which we can combine and separate into their constituents at will; to all intents and purposes, in their various reactions, they behave as elemental substances, and were it not that our analyses are able to resolve them, as the spectroscope resolves the nebulæ, we might well believe that here also we were dealing with simple primary elements. It is almost certain that great discoveries in this field of chemistry