72 PHYSICAL SCIENCE BK. n 



force as launched the bolt from the cloud causes it 

 2 to fall to the ground. Something of the same kind 

 happens to these celestial fires as to trees when 

 bent. The topmost branches if slender may be 

 dragged down so as to touch the ground ; but when 

 you let them go, they rebound to their original 

 position. You must not regard the condition which 

 an object involuntarily assumes as characteristic of 

 it. If you allow fire to go where it will, it will 

 return to the sky, the abode of all the lightest 

 bodies. But when there is anything to carry it 

 down and divert it from its natural course, that is 

 not a mark of its disposition but a token of its 

 subjection. 



XXV 



You and your friends say, an objector interposes, 

 that clouds emit fire through mutual friction 

 when they are moist, indeed wet. How can such 

 clouds produce fire, which is no more likely to be 

 generated by a cloud than by pure water ? 



XXVI 



i WELL, first of all, the fire which is thus produced is, 

 as it is found in the clouds, not water, but thick 

 air, adapted for the generating of water ; it is not yet 

 changed into it, but is already inclined toward, and 

 ready for, the change. There is no ground for 

 supposing that water is first gathered in the clouds 

 and afterwards shed from them. It falls simultane- 

 ously with its formation. But in the second place, 



