XII. _Lightning_, _thunder-bolt_, _thunder_. All agree that they occur in the clouds, but different explanations are given of their cause and relations. Anaxagoras connects them with the ether; Aristotle says they are due to exhalations of various kinds, from the earth, coming in contact with the clouds 62 XIII. The fire cannot be inherent in the clouds and fall from them. When it so comes it is forced 64 XIV. There is nothing inconsistent with this in the explanation given (I. xv.) of meteors. There may be an analogy with what is observed in cases of fire when isolated groups of houses take fire through gradual accumulation of heat 65 XV. Some (Stoics) think that spontaneous combustion takes place in the air 66 XVI. Difference between a flash of lightning and a bolt 66 XVII. Some explain the noise of thunder as due to hot meeting cold, as in the case of hot iron plunged into water 67 XVIII. Anaximander attributes the effects to air and explains all the phenomena by reference to it 67 XIX. Anaxagoras says it is the ether that acts on the lower atmosphere to produce them: it sends out fire 68 XX. Diogenes of Apollonia thinks that fire and air interact, producing one another, as may be observed in the various phenomena 68 XXI. Authorities discarded: independent explanation. A flash and a bolt are fire in some form: they differ only in degree 69 XXII. Analogy of fire on earth: it must apply above. Lightning due either to impact or friction. Hurricanes are a sufficient cause of the former 70 XXIII. Clouds and air may through friction also be a cause. The fires so produced are insubstantial and evanescent 71 XXIV. Fire by reason of its lightness levitates, just as water gravitates. But in the case of a bolt it is forced down, contrary to its nature, like a “weeping” tree 71 XXV. But it is said that wet clouds produce fire. How? 72 XXVI. There is no inconsistency in the combination in the same cloud of potential fire and water. A log burns at one end, exudes moisture at the other. An island on each of two occasions was thrown up by fire in the Aegean Sea, fire overcoming water. And clouds are, as a matter of fact, required for lightning: exceptions to this are only apparent 72 XXVII. Different kinds of thunder. The growling and the crashing, with their causes 75