When we consider the intense heat which has prevailed in Europe during July, and the circumstance that in America also the heat has been excessive, insomuch that in New York the number of deaths during the week ending July 6 was three times greater than the average, we are naturally led to the conclusion that the sun himself is giving out more heat than usual. Though not endorsing such an opinion, which, indeed, is not warranted by the facts, since terrestrial causes are quite sufficient to explain the recent unusual heats, we cannot refrain from noting, as at least a curious coincidence, that at the very time when the heat has been so great, the great central luminary of the solar system has been the scene of a very remarkable disturbance - an event, in fact, altogether unlike any which astronomers have hitherto observed.

Now certain Italian spectroscopists - Respighi, Sec-chi, Tacchini, and others - have set themselves the task of keeping a continual watch upon the chromato-sphere. They draw pictures of it, and of the mighty coloured prominences which are from time to time upreared out of, or through, the chromatospheric envelope. They note the vapours which are present, as well as what can be learned of the heat at which these vapours exist, their pressure, their rate of motion, and other like circumstances. It was while engaged in some of the more difficult and delicate of these tasks that Tacchini noticed the strange occurrence now to be described.

'1 have observed a phenomenon,' he says, 'which is altogether new in the whole series of my observations. Since May 6, I had found certain regions in the sun remarkable for the presence of magnesium.' Some of these extended half-way round the sun. This state of things continued, the extension of these magnesium regions gradually growing greater, until at length, 'on June 18,' says Tacchini, 'I was able to recognise the presence of magnesium quite round the sun-that is to say, the chromatosphere was completely invaded by the vapour of this metal. This ebullition was accompanied by an absence of the coloured prominences, while, on the contrary, the flames of the chromato-sphere were very marked and brilliant. It seemed to me as though I could see the surface of our great source of light renewing itself.' While this was going on Tacchini noticed (as had frequently happened before in his experience) that the bright streaks On the sun which are called faculae were particularly brilliant close to those parts of the edge of the disc where the flames of the chromatosphere were most splendid and characteristic. The granulations also, which the astronomer can recognise all over the sun, when a large telescope is employed, were unusually distinct.

Tacchini concludes (and the inference seems just) that there had not been a number of local eruptions of magnesium vapour, but complete expulsions. Only we would venture to substitute for the word 'expulsion' the expression 'outflow' or 'uprising,' since it may well be that these vapours rise by a quiet process resembling evaporation, and not by any action so violent that it could properly be regarded as expulsive.

In whatever way, however, the glowing vapour of magnesium thus streamed into the envelope of the sun, it would seem that the aspect of our luminary was modified by the process-not indeed in a very striking manner, or our observers in England would have noticed the change, yet appreciably. 'More than one person,' says Tacchini, 'has told me that the light of the sun has not at present its ordinary aspect; and at the Observatory we have judged that we might make the same remark. The change must be attributed to magnesium.'

It is impossible to consider attentively the remarkable occurrence recorded by Tacchini without being struck by the evidence which it affords of solar mutability. We know that during thousands of years our sun has poured forth his light and heat upon the worlds which circle around him, and that there has been no marked intermittence of the supply. We hear, indeed, of occasions when the sun has been darkened for a while; and we have abundant reasons for believing that he has at times been so spot-covered that there has been a notable diminution of the supply of light and heat for several days together. Yet we have had no reasons for anticipating that our sun might permanently lose so much of his heat and lustre that the inhabitants of earth would suffer. Tacchini's observation reminds us, however, that processes are at work upon the sun which admit of being checked or increased, interrupted altogether or exaggerated so violently, that the whole aspect of the sun, his condition as the fire and lamp of the planetary system, may be seriously affected.

If we only remember that our sun is one of the stars, not in any way distinguished, unless perhaps by relative insignificance, from the greater number of the stars which illuminate our skies at night or are revealed by the telescope, we shall learn to recognise the possibility that he may undergo marked changes. There are stars which after shining with apparent steadiness for thousands of years (possibly for millions of years before astronomy was thought of), have become suddenly much reduced in brightness, or after a few flickerings (as it were) have gone out altogether. There are others which have shone with equal steadiness, and have then suddenly blazed out for a while with a lustre exceeding a hundredfold that which they formerly possessed. It would be equally unpleasant for ourselves whether the sun suddenly lost the best part of his light, and presently went out altogether, or whether he suddenly grew fiftyfold brighter and hotter than he now is. Yet in the present position of sidereal astronomy, it is quite impossible to assert confidently that one event or the other might not take place at any time.

Fortunately, we may view this matter (just as astronomers have learned to view the prospect of mischievous collisions with comets) as a question of probabilities. Among so many thousands of stars there have been so many sudden outbursts of light and lire, so many sudden defalcations of splendour. Our sun is one of those thousands, and so far as we know takes his chance with the rest.

From the Spectator for August 1872.