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their chemical condition; and it is most remarkable that notwithstanding the chemical decompositions and recompositions that occur during the preparation of certain sorts, their final properties depend upon the original condition in which their · alkaline earths were found.

THE ERUPTIONS AT SANTORIN.*

We have on several occasions laid before our readers various facts concerning the interesting volcanic eruptions at Santorin, and we have now before us a valuable paper on the subject, accompanied by large and beautifully-executed maps and diagrams.

The German philosophers to whom we are indebted for this work commence by pointing out certain resemblances between the Kaimeni, or “burnt,” Islands of the Santorin group, and the volcanic region immediately surrounding and comprehending Vesuvius. The island of Thera, or Santorin, is approximately semicircular in form, and opposite to its western or concave side are two other islands, Therasia and Aspronisithe latter being very small-which follow the general curvature of the main island, and with it enclose a sca-basin more than five miles in diameter, in the midst of which the Kaimenis rise. Von Buch, in accordance with his well-known theory, considered the whole formation to be “a crater of elevation,formed by the upheaval of the sea-bed; but examination in this, as in most other cases, dissipates his conjectures, and shows these Santorin volcanoes to have modified the surface by the outpouring of molten inatter. MM. Fritsch, Reiss, and Stübel say, “Let us imagine Mount Vesuvius and Somma to be lowered, so that the sea might enter into and partly inundate the Atrio del Cavallo, we should then obtain a distribution of sea and land analogous to that seen at Thera and the Kaimeni Islands, a smaller part of the cone of Vesuvius rising from the sea in the midst of encircling Somma.” Somma is, as most of our readers will know, the name given to the ancient cone, the remains of which partially surround the newer cone known as Vesuvius, and formed in 79 during the tremendous eruption in which the elder Pliny lost his life. After indicating the analogies between Vesuvius and Santorin, our authors point out the differences, and observe that “while on Mount Vesuvius the volcanic action has always been confined to the existing

* "Santorin: the Kaimeni Islands.” From Observations by K. V. Fritsch, W. Reiss, and A. Stübel. Translated from the German.— Triibner and Co.

distinoruptions of"%by degrees is o irregularitily overflowing

crater, at least so far that they have never raised by its side any other mound approaching in height and extent to the great cone, we find, in the Gulf of Santorin, each separate revival of volcanic action, characterized by its separate and special formation, which we can trace as such even under water down to a common base. These formations owe their origin to a slow emission of large masses of lava quietly overflowing at their point of issue, filling up the irregularities of the bed of the sea, and rising by degrees as islands above the water-level. The eruptions of Mount Vesuvius, on the contrary, are mostly distinguished by a totally different character, inasmuch as the melted rock, flowing from a higher or lower point of eruption down the slope of the mountain cone, spreads in long but narrow streams."

An interesting peculiarity of the volcanic action at Santorin is the fight which the volcanic fires have had with the cold water of the sea. “ The quantity of steam sent forth at intervals of but a few minutes was so considerable, that it often rose to a column of more than 2000 metres in height.”* This magnificent display lasted for months, and acting upon the tough, viscid lava, assisted to produce the crater forms.

On the 16th of May, 1866, without previous symptoms of disturbance, two small islands appeared in the Kaimeni group. "No signs of anything occurring at the bottom of the sea had preceded this event, except that new soundings showed a depth less than that which had been previously observed in the channel.” The new islands, which looked like large heaps of black rock, increased from day to day, moving at the same time horizontaliy from north-west to south-east, as shown by accurate geometrical measurements. In April, the progress of the field of lava of Aphroessat was principally in a northerly direction, menacing thus to block up entirely the small harbour of St. George. In the beginning of May it became every day more apparent that the mass of lava had changed the direction of its onward movement, taking its course now to the south-west, in the direction of Palea Kaimeni.” By the 30th of May the new islands had increased to four, and they represented the emergent portions of the lava currents, which had filled a deep sea-trough between Nea and Palea Kaimeni.

The displacement of the May islands is highly curious. Our authors describe them as made up of wildly accumulated and brittle blocks, and they regard their onward movement as indicating “ not only a greater extension of the igneous mass at the bottom of the sea, but also a displacement and destruction of the yielding material in a much higher degree than that occasioned by the breakers.” In actions of this kind they find an explanation of the increase and decrease and total disappearance of such islands.

* Rather more than 2187 yards, or exceeding a mile and a quarter. † The name of a part of one of the islands.

The paper from which we have extracted the preceding information is illustrated by four large plates; the first is a reduction of the Admiralty chart of the Santorin group, with soundings of the adjacent waters; the second is a map, showing the successive enlargement of Nea Kaimeni; the third (called Plate II.) is a remarkably beautiful and interesting photograph of a model of the island and adjacent sea-bed, made by Herr Stübel; the fourth (called Plate III.) contains two fine photographs, one depicting a bird's-eye view of the island, previous to the eruption of 1866, and the other exhibiting their configu. ration after it, and showing the column of steam rising from the volcanic vents. These illustrations are very instructive, and will be highly esteemed by students of volcanic action.

JUPITER WITHOUT SATELLITES.

On the 21st of August the remarkable spectacle of Jupiter without his attendant satellites gratified the eyes of numerous observers. In London the weather was scarcely propitious, as a number of clouds were flitting slowly across the sky, and, at convenient hours, only occasional glimpses of the planet could be obtained. In some other localities a cloudless sky offered greater facilities, but those who were only favoured with intermittent views had much reason to be gratified with the singularity and beauty of the spectacle.

Of course the phrase, “ Jupiter without satellites,” is not literally true. The satellites had not forsaken their primary, but, by a series of remarkable coincidences, they all ceased for an hour and three-quarters to occupy visible positions at his sides ; so that, in any telescope not powerful enough to show the shadows, or the bodies of those that were on his disk, his luminous globe appeared wandering alone.

Jupiter is an enormous planet, the largest of our system, being 1300 times as big as our earth, and having a diameter of no less than 87,000 miles. Mr. Breen, in his “Planetary Worlds,” makes the following concise remarks respecting the four satellites of this wondrous globe. He says, “ The three inner satellites move all very nearly in the plane of the equator; but the fourth is slightly inclined to it. In consequence of this, and their proximity to Jupiter, the three first satellites are eclipsed at each revolution, which does not happen to the fourth, as it can pass above or below the shadow, Passing between Jupiter and the sun, they likewise produce solar eclipses,'and their shadows on the disk can be seen in the form of round dark spots, and in powerful telescopes even the satellites themselves are visible. Some very curious laws have been detected by the combination of the motions of the three interior satellites, and it is impossible that they can all be eclipsed at the same time. On some extraordinarily rare occasions, however, Jupiter has been seen without any satellites. They are not necessarily eclipsed on that account, as they may pass either before or behind the disk. The three interior satellites return to nearly the same position in respect to Jupiter in a period of 437 days 4 hours.”

Jupiter is composed of much lighter materials than our earth, his density being 243 when that of the Earth is reckoned at 1000; but from his great size he is able to exert a great power of attraction, and a body which would weigh 100 lbs. on the Earth would weigh 224 lbs. at Jupiter's equator, and as much as 276 at his poles. Jupiter's diurnal rotation takes place in a few minutes less than ten hours; so that his atmospheric currents resulting from rotation must be extremely violent-a fact which helps to explain the occasional rapid changes in his belts, which are supposed to be the body of the planet seen through his clouds, but which makes it more difficult to account for the frequent persistence of those belts in nearly the same condition for considerable periods. The first of Jupiter's satellites has a diameter of 2400, and is about the density of water; the second satellite is somewhat less in bulk, being 2190 miles in diameter, and thus resembles our moon in size : its density is greater than that of the other three, or of the planet himself; the third satellite is 3580 miles in diameter; and the fourth 3060 miles. Their distances from Jupiter's centre are, respectively, 278,542 miles, 442,904. miles, 706,714 miles, and 1,242,619. They revolve round theirprimary in periods varying from 1 day 18h. 27m. 33.505s., the time of the first, to 16 days 16h. 32m. 11•271s., the time of the fourth.

From the preceding facts, it will be seen why the positions of the satellites change so constantly and so quickly. On the 21st of August their motions were so curiously combined, that. three satellites were on the disk of the planet at the same time, and a fourth behind it. The appearance of the first satellite gliding on to the disk at 10.4 P.M. was extremely beautiful as we observed it. The first contact, the gradual passage, the slight addition to the planet's margin as the last portion of the satellite's disk passed inwards, was beautifully shown in one of

VOL. XII.—NO. II.

the Browning-With silvered-mirror telescopes, with a power of 120.

When the cloudless intervals permitted, the shadows of the three satellites (1, 3, and 4) on the disk were sharp, and as black as ink, contrasting boldly with the brilliant portions of the planet, and making the coppery tint of the belts more conspicuous by their deviation from the much darker shadow tones. Fortunate observers saw the entrance of the third satellite on Jupiter's disk at 8.14 P.M., eclipse of the second satellite, the entry of the fourth and first on the disk, preceded by their shadows, the passage off the disk of these satellites and their shadows, and also the reappearance of the second satellite from behind Jupiter. Few could expect to be so lucky as to see all the incidents, which a perfect continuance of fine weather could disclose; but a great many saw enough to make the evening of the 21st of August memorable in their astronomical experience. Near Dublin, the Hon. Mrs. Ward informs us the sky was propitious for nearly the whole time, though clouded when the fourth satellite left the disk, and the series of phenomena ended.

IRRITABILITY OF PLANTS.

BY M. CH. BLONDEAU.

(From " Comptes Rendus.") We have undertaken, in the course of this year, a number of experiments on the irritability of plants, which show that the faculty which some of them, and especially the sensitive plant, possesses of executing movements which appear voluntary, may be suspended by many agents, such as ether, chloroform, carbonic oxide, and spirit of turpentine-all of which are known to act upon the nervous system of animals.

Amongst these experiments, one appears to us of sufficient interest to deserve the attention of the Academy of Science; it relates to the action of the electric current on the Mimosa pudica (sensitive plant).

We selected four plants perfectly developed, and so sensi. tive that the slightest contact, such as the friction of a fly's wing, caused their leaves to shut up, and the petioles (leaf stalks) to droop along their stems. Placing the pots containing these plants on an insulating stand, we attached to the two ends of their stems a small copper wire in order to pass through them the current from a single Bunsen's cell. After a few seconds, when the plants reopened their leaves, and erected

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