7 8 PHYSICAL SCIENCE BK. n 



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 



i 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 



