When the air is rarefied by heat, its coercive power is diminished, so that the electricity escapes from the clouds in those lambent diffuse flashes without thunder so frequent in warm summer evenings; and when the atmosphere is highly charged with electricity, it not unfrequently happens that electric light, in the form of a star, is seen on the topmasts and yard-arms of ships. In 1831 the French officers at Algiers were surprised to see brushes of light on the heads of their comrades, and at the points of their fingers when they held up their hands. This phenomenon was well known to the ancients, who reckoned it a lucky omen. Many substances, in decaying, emit light, which is attributed to electricity, such as fish and rotten wood. Oyster-shells, and a variety of minerals, become phosphorescent at certain temperatures when exposed to electric shocks or friction: indeed, most of the causes which disturb molecular equilibrium give rise to phosphoric phenomena. The minerals possessing this property are generally coloured or imperfectly transparent; and, though the colour of this light varies in different substances, it has no fixed relation to the colour of the mineral. An intense heat entirely destroys this property, and the phosphorescent light developed by heat has no connexion with light produced by friction; for Sir David Brewster observed that bodies deprived of the faculty of emitting the one are still capable of giving out the other. Among the bodies which generally become phosphorescent when exposed to heat, there are some specimens which do not possess this property; wherefore phosphorescence cannot be regarded as an essential character of the minerals possessing it. Sulphuret of calcium, known as Canton’s phosphorus, and the sulphuret of barium, or Bologna stone, possess the phosphorescent property in an eminent degree. Multitudes of fish are endowed with the power of emitting light at pleasure, no doubt to enable them to pursue their prey at depths where the sunbeams cannot penetrate. Flashes of light are frequently seen to dart along a shoal of herrings or pilchards; and the Medusa tribes are noted for their phosphorescent brilliancy, many of which are extremely small, and so numerous as to make the wake of a vessel look like a stream of silver. Nevertheless, the luminous appearance which is frequently observed in the sea during the summer months cannot always be attributed to marine animalculæ, as the following narrative will show:—