jets and clouds of hydrogen cooler than the luminous prominences, and so looking black when projected on a background of the hotter gas. Tacchini and Respighi have kept up a careful and systematic record of chromospheric phenomena in a statistical way.

During the past four years the most important investigations upon the solar radiation have been those of Langley. The results of the Mount Whitney Expedition of 1881, with those of certain supplementary investigations, were published in 1884. They fully confirm his earlier conclusion, that the previously received value of the solar constant should be largely increased, and from his data he fixes it at about thirty calories per square metre per minute instead of twentyfive. Of course, this involves a corresponding increase of twenty per cent in all the figures which are given to illustrate the immensity of the solar radiation in various ways.

It is worth noting also that Langley, following many French authorities, prefers to employ a smaller heat-unit than the calory used by the writer-the gramme-degree instead of the kilogramme-degree. His solar constant is the number of these small calories received per minute upon a square centimetre, and therefore on his scale of notation stands as three instead of thirty. We mention it, because this discordance in the definition of the calory has led to some confusion among those not entirely familiar with the subject. If we were to follow strictly the so-called c. g. s. system, the solar constant would be represented by a number still sixty times smaller-viz., 0·050 (grammedegrees per centimetre per second).

Professor Langley has also, with the bolometer, carried the investigation of the invisible portion of the sun's heat-spectrum far beyond any of his predecessors, and has discovered and measured the wavelength of a large number of absorption-bands in this region; his results are confirmed by those of Becquerel and Abney, the latter operating by means of photography, and the former by means of the effect of the invisible infra-red radiations in quenching the phosphorescence of a suitably prepared screen.

Langley finds that the solar spectrum seems to terminate abruptly at a wave-length of about thirty thousand on Ångström's scale: he does not find in the sun's heat any of the long-waved, slowly-pulsating radiations, such as are emitted by bodies at or below the temperature of boiling water. We might think that they had been absorbed in the earth's atmosphere, were it not that he finds just these rays relatively abundant in the spectrum of lunar heat. He also finds them in the heat-spectrum of the electric arc, so that it is difficult to suppose that they do not originally exist in the solar spectrum. Unless there is some hidden fallacy or error in some of the observations, we are almost driven to admit that they have been absorbed in interplanetary space. But probably it will be best to await further confirmation of the experimental results before accepting so remarkable a conclusion.

At the eclipse of 1883, observed on Caroline Island, in the Pacific Ocean, by French and American parties, Professor Hastings made observations for the purpose of testing a theory he had framed that the outlying regions of the corona are merely a diffraction effect produced by the edge of the moon; the diffraction being not that due to the regular periodicity of light-vibrations, ordinarily discussed, but due to the probable continually occurring discontinuity or change of phase in the vibrations. It seems probable, from a not perfectly complete investigation, that such discontinuity might scatter light far beyond the limits of ordinary diffraction. He found during the eclipse, by an apparatus constructed expressly for the purpose, that the bright corona-line (1474 K) was always visible to a much greater distance from the sun on the side least deeply covered by the moon than on the other, as unquestionably ought to be the case if his theory were correct.

But the same thing would result from the diffusion of light by the air; and the French observers, and nearly all others who have discussed the matter, feel satisfied that this is the true explanation of what he saw. He himself now, we understand, thinks it not impossible that a thin cloud may have passed over the sun just at the critical moment, and so have vitiated his observation.

The discussion which has followed his publication seems to have only strengthened the older view, that the corona is a true solar appendage, an intensely luminous though inconceivably attenuated cloud of gas, fog, and dust, surrounding the sun, formed and shaped by solar forces.

The fact that comets, themselves mere airy nothings, have several times (the last instance was in 1882) passed absolutely through the corona without experiencing any sensible disturbance of path or structure, has, however, been always felt by many as an almost insuperable difficulty with this accepted theory, and more than anything else led Professor Hastings to propose his new hypothesis. But, on careful consideration, we shall find that our conceptions of the possible attenuation of shining matter near the sun will bear all the needed stretching without involving any absurdity. Recalling the phenomena of the electrical discharge in Crookes's tubes, it is clear that a "cloud," with perhaps only a single molecule to the cubic foot (but thousands of miles in thickness), would answer every luminous condition of the phenomena. And all the rifts and streamers, and all the peculiar structure and curved details of form, cry out against the diffraction hypothesis.

At present the most interesting debate upon the subject centers around the attempts of Dr. Huggins (first in 1883) to obtain photographs of the corona in full sunlight. He succeeded in getting a number of plates showing around the sun certain faint, elusive halo-forms which certainly look very coronal. Plans were made, and were carried out, in 1884, for using a similar apparatus upon the Riffelberg

in Switzerland, and since then at the Cape of Good Hope. So far nothing has been obtained, however, much in advance of Dr. Huggins's own first results. But since September, 1883, until very recently, the air, as every one knows, has been full of a fine haze, probably composed in the main of dust and vapor from Krakatoa, which has greatly interfered with all such operations. It is now fast clearing away, and, if Dr. Huggins's views are correct, it is reasonable to expect that a much greater measure of success will be reached next winter at the Cape, and perhaps during the present summer in England and Switzerland.

About the same time that Dr. Huggins was photographing in England, Professor Wright, of New Haven, was experimenting on the same subject in a different way. He reflected the sun's rays into a darkened room by a heliostat, cut out all but the blue and violet rays by a suitable absorbing-cell, and then formed an image of the sun and its surroundings upon a sensitive fluorescent screen, stopping out the sun's disk itself. He obtained on the screen, on more than one occasion, what he then believed and still believes to be a true image of the corona. But the aërial haze soon intervened to put an end to all such operations; for of course it is evident that success, whether by photography or by fluorescence, is possible only under conditions of unexceptionable atmospheric purity.

Both Professor Wright and Dr. Huggins base their hopes upon the belief, which seems to be warranted by the spectrum-photographs obtained during the Egyptian eclipse of May, 1882, that the light of the corona and of the upper regions of the sun's "atmosphere" is peculiarly rich in violet and ultra-violet rays-that the corona is far more brilliant to the photographic plate and to the fluorescent screen than to the eye.

Probably it must be admitted that at present the predominant opinion among astronomers and photographers is against the practicability of reaching the corona without an eclipse, by any such methods; at the same time, to the writer at least, the case appears by no means hopeless, and success is certainly most devoutly to be desired.

P. S.-The reports from the recent eclipse of August 29th, observed by British and American parties on the Island of Grenada, in the Southern West Indies, have just come to hand, and are strongly unfavorable to the reality of the coronal appearances obtained by Huggins and Wright in their attempts to render the corona visible without an eclipse.

Plates furnished by Mr. Huggins, and precisely similar to those which he has employed in his photographic experiments, were exposed by Captain Darwin during the totality (as well as before and after it), in an apparatus like Mr. Huggins's, with a time of exposure the same that he has been using, and were treated and developed according to his directions. The plates exposed during the totality show no corona

at all, the exposure proving insufficient to bring it out. Of course, this makes it extremely probable that what looks like the corona upon plates exposed to the uneclipsed sun is merely a fallacious ghost, due, as his opponents have always claimed, to something in his apparatus or process, or else to the scattered light in our atmosphere.

It is true, as Mr. Common points out, that the air was by no means satisfactorily clear during the eclipse, and the result, therefore, is not absolutely conclusive; but it must be conceded, and Mr. Huggins himself admits it, that the probability is now heavily against him.

Captain Darwin obtained good pictures of the corona with ordinary plates exposed for a longer time in the usual apparatus. October 1, 1886.

CHEVREUL AT A HUNDRED.

BY WILLIAM H. LARRABEE.

HE occasion of M. Chevreul's completing the one hundredth year

with appropriate observances and honors. The festivities were begun in the National Society of Agriculture, whose custom it has been to elect M. Chevreul its president every other year. A committee of this society had been formed in April, under the presidency of M. Charles Brongniart, and had collected the sum of fifteen thousand francs for the purpose of striking a commemorative medal for the centenary. Addresses were delivered by Deputy Louis Passy, and, in presenting the medal, by M. Brongniart, who assured M. Chevreul that he was the object of the respect and admiration of all civilized nations. M. Chevreul replied: "All that I have heard causes me much embarrassment. And why? On account of the warmth of the profound and numerous sentiments which you have expressed. I never anticipated the honor that my comrades have paid me."

In the Academy of Sciences, whose regular meeting took place on the 30th, M. Blanchard, in the absence from Paris of President Admiral Jurien de la Gravière, took the chair and made the Academy's address. He remarked upon the session's occurring on that day, as if the hour had been chosen for the event, saying that "in the family it is on the eve of the marked day that the festival is given was it not fitting that it should be the same in the Academy, our intellectual family, which we love more and more as we grow older?" He referred to the fact that he had, as President of the Academy, predicted this very event three years before, when M. Chevreul was entering upon his ninety-eighth year. Then, having made a general mention of the value of M. Chevreul's discoveries, he said: "The investigator, absorbed in his mission, dreams of nothing but of extending its domain. If he succeeds in unveiling facts of considerable inter

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est, he deserves well of science. If an application capable of furnishing the country a new source of wealth arises out of his labors, it is a glory to him; but the man of science finds his highest recompense when he has succeeded in spreading a comfort through the nation, and procuring for those who are disinherited of this world's goods a little of the luxury which it had seemed could only be obtained with wealth. Master, experimenter, philosopher, Monsieur Chevreul, you

have known all of these triumphs. Again I tell you, nothing is want-ing to the fullness of your life. By your stories of remote events of which you have been a witness, you have charmed those who in age might be your sons, and those, still more numerous, who might be your grandsons. Your memory, yet in its freshness, permits you still to instruct those who might be your great-grandchildren. After tomorrow, you will count the days, weeks, and years of your new century. That the years may be many is the wish of your fellows and admirers." M. Chevreul in reply cited as a proof that a man's repu tation depends greatly upon the trial of time, the examples of Newton and Leibnitz, the latter of whom said, "Seek first the demonstration of causes.' "Newton preferred the more fruitful idea, 'Seek the cause of phenomena, and when you have found it inquire what is the cause of that cause.' There is a wide gulf between the two formulas. To my mind Newton is greater than Leibnitz. Time has proved it."

Representative scientific men of other countries, com