In Greece they come chiefly in spring and autumn. It is noticeable that in all latitudes they come most often in the afternoon. CHAPTER III THE FLASH AND THE SOUND The Romans attributed a mysterious influence to each manifestation of electricity. They divided lightning into individual and family lightning, lightning of advice, monitory, explanatory, expostulatory, confirmatory, auxiliary, disagreeable, perfidious, pestiferous, menacing, murderous, etc., etc. They adapted it to every taste and circumstance, but modern science has come to put order into this Capharnaum. When a cloud is superabundantly charged with electricity, this electricity, which is compressed in the cloudy envelope, tries to escape in order to join the electricity accumulated either in another cloud or on the ground. An electric deflagration ensues, and a long ignited dart precipitates itself into space, showing us on a large scale what our experience of physics has taught us in a small way in our laboratories. This luminous and often dazzling trail constitutes lightning. Lightning is not always the same, and in order to classify the different forms it takes more easily, it can be divided into three groups--diffused lightning, linear lightning, and fireballs. This last is the most curious of the three. The variety and eccentricity of fireballs are celebrated in the history of lightning, and I propose to devote the following chapter to their vagaries. Diffused lightning is the commonest of all. You can count hundreds of flashes on a stormy night. Occasionally they succeed one another with such rapidity that the sky is momentarily entirely illumined with a fantastic brightness. At these times great sombre clouds can be seen surging from the darkness of the night, to shine suddenly with an ephemeral brightness of a diffused red, blue, or violet tinge. Their irregular shapes, with their jagged edges of light, are visible against the dark background of the heavens, and the thunder growls monotonously. Whether the exchange of electricity is produced on a vast stretch between two rows of clouds, or whether it is manifested by a long thin spark launched like an arrow and veiled by the curtain of clouds, all that can be seen is a strange light, vague, diaphanous, instantaneous, which sometimes spreads itself like a sheet of fire all over the horizon. It is diffused lightning which gives us the finest storm effects on those heavy summer evenings when the air is breathless and saturated with electricity. Suddenly the clouds are illumined, nebulous veils of light on which can be seen, in sombre fantastic, fugitive vision, the outlines of the trees, houses, and other landmarks. Then, all at once, heaven and earth fall back into a darkness deeper than before, owing to the contrast.