On Sal Nitrum and Nitro- Aerial Spirit 1 5 r 



it descended to the earth in that way) could imprison 

 inflammable matter with force sufficient for the stroke 

 of a thunderbolt. 



Wherefore we may assume that the immense 

 force of lightning is produced in the following way. 

 For instance, if a tower or other obstacle stands, at any 

 time, directly in the path of the rush of air (for it is to 

 be observed that, in a thunderstorm, the air pressed 

 by the descending cloud spreads out one way or other), 

 it happens that the air is brought to a stand by 

 the opposition of the said things and is greatly con- 

 densed, as is shown in Plate I., Fig. 10. And not only 

 the aerial particles but the sulphureous also, which 

 are raised into the air by the great heat of the sun, are 

 densely collected near the obstacle. But in conse- 

 quence of the dense collection of the particles of both 

 kinds, and their igniting in the manner aforesaid on 

 account of the violent concussion of the air, a globular 

 fire, and that very impetuous and rivalling gunpowder, 

 is produced. For it has been shown elsewhere that 

 the force of gunpowder is caused by nitro-aerial 

 particles bursting out in densest crowd from the 

 ignited nitre. Accordingly, since aerial particles are 

 charged with the same nitro-aerial particles, if they 

 ignite when densely collected, the flame produced by 

 them will be very impetuous for the very same reason 

 as in the case of gunpowder ; so that now it is no 

 wonder that a thunderbolt sometimes overwhelms 

 and prostrates whatever it meets. 



In what has been said, we must look for the reason 

 why a thunderbolt sometimes melts a sword, while 

 the sheath is left intact. No doubt, since the motion 

 of the air excited in thunder shakes even solid bodies, 

 it sometimes happens that iron and other solid bodies 

 against which the force of that motion is specially 



