Linear lightning is more terrible. It is regarded by astronomers as the most perfect form of destructive lightning. It is a strong flash--a thin trail of light--very clear, and extraordinarily rapid, which shoots from an electric cloud to the earth, or from one cloud to another. Like a supple and undulating serpent of fire, it twists itself luminously into space, spreading itself menacingly in the heavens with its long spirals of light. Sometimes--in a hurry, no doubt, to reach its prey--it effects its passage in a straight line, but as a rule it follows a sinuous track, and forms itself into a zigzag at an obtuse angle. The different forms which this lightning takes are no doubt attributable to various causes. One of the chief of these seems to be the unequal distribution of humidity in the air, which renders it a more or less good conductor. In fact, fulminic matter is strongly attracted towards damp regions, and goes quickly from one point to another, guided in its chosen way by the hygrometrical conditions of the atmosphere; and it is these constant changes of direction which determine the meanderings of its course. Thus the lightning would trace a sort of plan of the hygrometrical state of the air for a certain portion of the atmosphere. For it, the short road is hardly ever the straight line. On the other hand, the variability of the overloading of electricity has something to say to the form it takes. Sometimes lightning forms itself into two or three branches, and becomes forked lightning. Or it even divides itself into a number of points from a principal branch, out of which a great many sparks burst forth. These incandescent sheaves move through space with extraordinary agility. It has not been possible to measure their speed with absolute accuracy, but their rapidity is such that their transit appears to be instantaneous. The latest researches seem to have proved that their speed is superior to that of light, which is 300,000 kilometres a second. Lightning is not always of a dazzling whiteness, it is often yellow, red, blue, violet, or green. Its colour depends on the quantity of the electricity thrown on the atmosphere by the discharge; on the density of the air at the time of the passage of the ignited matter; on its hygrometrical state, and on the substances which it contains during suspension. It has been remarked in the study of physics that the electric spark is white in the open air, but that it gets a violet tinge in the vacuum of a pneumatic machine. This proves that violet lightning comes from the far-off regions of the atmosphere. It traverses a bed of rarified air, and shows the great height of the storm-clouds from which it emanates.