68 PHYSICAL SCIENCE BK. n 



no lightning ? The rarity and feebleness of the air 

 render it incapable of producing flame, while yet 

 sufficient to produce sound. Lightning, according 

 to him, then, is really a disturbance where the 

 atmosphere is merely parted and rushes hither and 

 thither, displaying a faint fire that will not issue 

 from its place. As for the thunderbolt, it is the 

 career of the more active and denser air. 



XIX 



1 ANAXAGORAS says all the phenomena correspond 

 to the descent of some force from the ether to the 



2 lower regions. So when the fire encounters cold 

 clouds it emits a sound ; when it cleaves them there 

 is a flash ; less violence in the fires produces 

 lightning, greater, thunderbolts. 



XX 



1 DIOGENES of Apollonia asserts that thunder arises 

 in some cases from fire, in some from air. Fire 

 precedes those it produces, to herald them. Those 

 that are attended with rattling noise, but without 

 flash, are produced by air. Either sound or flash, 

 I grant, can and sometimes does occur without the 

 other. Still, their powers are not distinct, each 

 may be produced by each. For will any one say 

 that air borne with great violence, when it can 



2 produce sound, will not also produce fire ? Will 

 not every one grant, too, that fire as well as air 

 may sometimes burst the clouds without darting 

 from them, for example, if it has burst through a 



