78 PHYSICAL SCIENCE BK. 11 



by the concourse of dry bodies, not of clouds : with 

 such a glow in the firmament there probably were 



2 no clouds at all. Cambyses once sent an army to 

 the temple of Jupiter Ammon in the desert. The 

 sand raised by the south wind fell on it like snow- 

 flakes, first covering and finally overwhelming it. 

 Probably on that occasion also there was thunder 

 and lightning, caused by the mutual friction of 

 the particles of sand. Such a view is not in- 



3 consistent with my contention above. I have said 

 that the earth s exhalations contain bodies of two 

 kinds, dry and moist, portions of which roam 

 through the whole expanse of the atmosphere. So 

 if any heavy element be introduced, it makes a 

 cloud thicker and more solid than if its texture 

 were of pure air exclusively. Such a [solid] cloud 



4 may burst with a loud report. The elements I 

 have mentioned, whether they have charged the 

 atmosphere with moist fires or with earth-sweeping 

 winds, must produce a cloud before they produce 

 a report. Dry elements no less than moist may 

 make up a cloud. For cloud, as we have already 

 said, is just a condensation of thick air. 



XXXI 



BUT further, if you will but open your eyes to 

 them, there are marvellous effects in lightning that 

 leave no doubt that a subtle divine power is 

 inherent in it. For example, coins are fused while 

 the purse containing them is uninjured and intact. 

 A sword is melted while the sheath remains. The 

 iron point is fused in a javelin, but the wooden 

 shaft suffers no damage. The jar is smashed and 



