CHAPTER XVIII. ECLIPSE--VISITS OF SCIENTIFIC MEN--LIFE AT NAPLES--DARWIN'S BOOKS--REMARKS ON CIVILIZATION--FINE AURORA BOREALIS--DEATH OF HERSCHEL--SUMMER AT SORRENTO--BILL FOR PROTECTION OF ANIMALS--NINETY-SECOND YEAR--LETTER FROM PROFESSOR SEDGWICK--GRAND ERUPTION OF VESUVIUS--LAST SUMMER AT SORRENTO, PLANTS FOUND THERE--CONCLUSION. The summer of 1870 was unusually cool; but the winter has been extremely gloomy, with torrents of rain, and occasionally such thick fogs, that I could see neither to read nor to write. We had no storms during the hot weather; but on the afternoon of the 21st December, there was one of the finest thunderstorms I ever saw; the lightning was intensely vivid, and took the strangest forms, darting in all directions through the air before it struck, and sometimes darting from the ground or the sea to the clouds. It ended in a deluge of rain, which lasted all night, and made us augur ill for the solar eclipse next day; and, sure enough, when I awoke next morning, the sky was darkened by clouds and rain. Fortunately, it cleared up just as the eclipse began; we were all prepared for observing it, and we followed its progress through the opening in the clouds till at last there was only a very slender crescent of the sun's disc left; its convexity was turned upwards, and its horns were nearly horizontal. It was then hidden by a dense mass of clouds; but after a time they opened, and I saw the edge of the moon leave the limb of the sun. The appearance of the landscape was very lurid, but by no means very dark. The common people and children had a very good view of the eclipse, reflected by the pools of water in the streets. Many of the astronomers who had been in Sicily observing the eclipse came to see me as they passed through Naples. One of their principal objects was to ascertain the nature of the corona, or bright white rays which surround the dark lunar disc at the time of the greatest obscurity. The spectroscope showed that it was decidedly auroral, but as the aurora was seen on the dark disc of the moon it must have been due to the earth's atmosphere. Part of the corona was polarized, and consequently must have been material; the question is, Can it be the ethereal medium? A question of immense importance, since the whole theory of light and colours and the resistance of Encke's comet depends upon that hypothesis. The question is still in abeyance, but I have no doubt that it will be decided in the affirmative, and that even the cause of gravitation will be known eventually.