Such is Dr. Huggins’ account; but it is manifest that this comet underwent important changes, of which—we are surprised to note—Dr. Huggins has taken no account. Thus, in April, Professors Tacchini and Vogel found simply a faint continuous spectrum. In May, Vogel found that the three bands associated with carbon were present, though faint, while there was no trace of the sodium band. On the contrary, on the nights of June 4, 5, and 7, Dr. B. Hasselberg, of the Observatory of Pulkowa, found that the nucleus of the comet gave a very strong and extended continuous spectrum, with an “excessively strong” bright line in the orange yellow, proved by micrometrical measurement to be identical with the D line—the well known double sodium line of the solar spectrum. The observation was confirmed by Dunér, Bredichin, and Vogel. On this Mr. Hind remarks, “It is necessary to conclude that, during the last fortnight of May, the spectrum of Wells’ comet had changed in a manner of which the history of science furnishes no precedent.” It must, however, be remembered that as yet no comets have been examined under sufficiently favorable conditions, to enable us to say whether the change thus observed was really exceptional, or only exceptional in being for the first time noted. Whenever such a comet as Donati’s comes favorably under spectroscopic scrutiny, we shall probably learn something about these changes which will throw more light than anything yet discovered on the physical economy of these mysterious bodies. What, then, do we know certainly respecting comets? What may we surmise with more or less probability? And in what direction may we look with most hope for future information? We know certainly that, in whatever way they are formed, the sun excites intense disturbance in them as they approach him. Prof. Stokes remarks that these effects, so much greater at a first view than we might fairly expect in the case of many of the comets observed, which have approached the sun no nearer than our own earth does, or not so near, may be accounted for by the circumstance that comets travel in what may be regarded as, to all intents and purposes, a vacuum. From Dr. Crooke’s experiments on very high vacua, we may infer that there is very little loss of heat, except by radiation. Thus the heat received by the meteoric components of a comet would be much greater than might otherwise be expected. Dr. Huggins mentions, in the same connection, the remarkable persistence of the bright trains of meteors in the rare upper air, which sometimes remain visible for three-quarters of an hour before the light fades, as the heat is gradually radiated away. “Our reasoning on these points,” he remarks, in his dry way, “would undergo considerable modification if we accept the views as to the condition of interplanetary space and of the sun’s action which have been recently suggested by Dr. Siemens in his solar theory”—but of course we do not.