ELECTRICITY 157 



"Go on? Here is where difficulty begins. Do you 

 know, my children, that your questions are some- 

 times very embarrassing ! ' Go on ' is soon said ; and, 

 filled with unbounded faith in your Uncle Paul's 

 knowledge, you expect an answer which, you feel 

 smv, will satisfy your curiosity. You must, how- 

 ever, understand that there are innumerable things 

 beyond your intelligence, and before you can grasp 

 them you must attain to riper reason. With age and 

 study many things will become clear that now are 

 dark to you. In this number is the cause of thunder. 

 I am very willing to tell you something about it ; but 

 if you do not understand all that I say you must 

 blame your own premature curiosity. It is a difficult 

 subject for you, very difficult. " 



"Only tell us about it," Jules persisted; "we will 

 listen attentively." 



"So belt. Air is not visible, one cannot take hold 

 of it ; if it were always at rest you would not, per- 

 haps, suspect its existence. But when a violent wind 

 bends tall poplars and scatters the leaves in eddies, 

 when it uproots trees and carries off the roofs of 

 buildings, who can doubt the existence of air! For 

 wind is only air streaming irresistibly from place to 

 place. Air, so subtle, so invisible, so peaceful in re- 

 pose, is therefore in very truth a material substance, 

 even a very brutal one when in violent motion. That 

 is to say, a substance can exist, although at times 

 nothing betrays its presence. We do not see it or 

 touch it, are not sensible of it, and yet it is there, all 

 about us ; we are surrounded by it, live in the midst 

 of it. 



