The tremendous heat to which many comets are subjected during perihelion passage is an important point for consideration, in attempting to form an opinion of the physical structure of comets. Newton calculated that the comet of 1680 was subjected to a heat 2,000 times greater than that of red-hot iron. But comets have been known to approach the sun even more closely. Sir John Herschel estimates that the comet of 1843 was subjected to a heat exceeding in the proportion of 24 1/2 to 1 the heat concentrated in the focus of Perkins' great lens. Yet the heat thus concentrated had sufficed to melt agate, rock-crystal, and cornelian.

We cannot wonder that so great an intensity of heat should have produced remarkable effects upon many comets. The great wonder is that any comet should resist the effects of such heat without being dissipated into space.

We learn from Seneca that Ephorus, an ancient Greek author, mentions a comet which divided into two distinct comets. Kepler considered that two comets which were seen together in 1618 had been produced by the division of a single comet. Cysatus noticed that the great comet of 1618 showed obvious signs of a tendency to break up into fragments. This comet when first seen appeared as a circular nebulous cloud. A few weeks later it seemed to be divided into several distinct cloudlike masses. On December 20 ' it resembled a multitude of small stars.'

We might doubt whether these observations were entitled to credit were it not that, quite recently, Biela's comet has been seen to separate into two distinct comets, each having a nucleus, coma, and tail, and each of which pursued its course independently until distance concealed both from view.

It is clear that nothing but a long series of careful observations can put us in a position to theorise with confidence, respecting the nature of comets, the processes of change which they undergo, and the functions which they subserve in the economy of the solar system. We may therefore dwell with particular satisfaction on the fact that every comet which has appeared during the last two years has been subjected to careful observation, and that at length, by means of spectroscopic analysis we are beginning to get hold of positive facts respecting comets, and have promise of shortly being able to form consistent theories with regard to these singular members of the solar system.

I have had occasion in other works to speak of the principles on which spectroscopic analysis depends; but I think it best briefly to restate the most important points. When the light from a luminous object is received upon a prism, there is formed what is called the prismatic spectrum. According to the nature of the source of light this spectrum varies in appearance. If the source of light is an incandescent body the spectrum is a continuous, rainbow-tinted streak. Where the 1ight comes from an incandescent mass surrounded with cooler vapours, the streak of rainbowcoloured light is crossed by dark lines whose position indicates the nature of the vapours which the light has traversed. When the light comes from luminous vapours the spectrum consists wholly of bright lines; and these have exactly the same position as the corresponding dark lines which are seen when the same vapours intercept light from an incandescent solid mass. Lastly, when light is reflected from an opaque substance, the spectrum is the same as that which would be presented by the light before reflection, unless the opaque substance is surrounded by vapours, in which case the spectrum will be crossed by new dark lines corresponding to the absorptive qualities exerted by those particular vapours.

We see then the wonderful qualities of the new analysis. Applied to the sun and stars it has enabled our physicists and astronomers to pronounce as confidently that certain elements exist in these far distant orbs, as the chemist can pronounce on the constitution of substances submitted to his direct analysis. The questions, or some of them, which have been at issue respecting comets, will undoubtedly yield to the powers of the spectroscope. The great want, at present, is a brilliant comet to work upon. Donati's comet (1859), or the great comet of 1861 would have served this purpose admirably, but the first came in the very year in which the principles of spectroscopic analysis were first discovered; and the powers of the spectroscope were only just beginning to be recognised when the comet of 1861 made its brief visit to our northern skies.

Two small comets have been analysed with the spectroscope, and each presented similar results. The spectrum in each case consisted of thin bright lines on a faint continuous streak of light. And from the fact that the bright lines did not extend across the whole breadth of the faint streak of light, it became evident that they formed the spectrum of the nucleus, the faint continuous spectrum belonging to the coma. Hence it resulted that the nucleus of each of these small comets consisted of self-luminous gas, while the coma either consisted of incandescent solid matter or shone by reflecting the light of the sun. The latter is far the more probable hypothesis. In fact, when we consider the extreme tenuity of the substance of a comet, and therefore the certainty that if composed of solid matter such matter must be dispersed in very minute fragments, we shall recognise the extreme improbability that these fragments should be self-luminous through intensity of heat. If the comets had been brighter, I may remark, there would have been no dubiety respecting this point, since it would have been possible to compare the continuous streak of light with the solar spectrum, and by the resemblance or dissimilarity of the two spectra, to determine whether the coma really shines by reflecting the sun's light or not.

Brorsen's comet has now been examined with the spectroscope, and with results quite different from those which attended the analysis of the other two. Dr. Huggins, the physicist, who examined the latter, says of Brorsen's comet: