-1859] WRITINGS OF JOSEPH HENRY. 345 



formation of the nitric acid we have mentioned, and with- 

 out such an explanation, it would be difficult to conceive 

 how particles of oxygen and nitrogen, which are rendered 

 mutually repulsive by the electrical discharge, should enter 

 into chemical combination. 



We have seen that though metals are generally good con- 

 ductors, yet when electricity falls upon a rod of iron or cop- 

 per explosively, the energetic repulsion, which must always 

 accompany these explosions, tends to throw the particles off 

 on all sides, and when the discharge is sufficiently great 

 the conductor itself is dissipated in vapor. Water is a much 

 inferior conductor to iron, and though a large mass of it will 

 silently discharge a conductor, yet it offers great resistance 

 to the transmission of electricity explosively, and hence the 

 electricity is sometimes seen to leave a conductor, and pass 

 a considerable distance over the surface of water, rather than 

 to force its passage through the interior of the mass. It is 

 therefore highly important in arranging lightning rods that 

 they should be connected at the lower end with a large sur- 

 face of conducting matter, to prevent as far as possible the 

 fluid from leaving the rod in the case of an explosive dis- 

 charge. 



Electricity of the Atmosphere. 



Having given in the preceding sections a brief exposition 

 of the general principles of electricity, we are now prepared 

 to apply these to an exposition of the phenomena of atmos- 

 pheric electricity. 



The origin of the electricity of the atmosphere has long 

 occupied the attention of physicists, and at different times 

 they have apparently settled down on some plausible hypoth- 

 esis which merely offered a probable explanation of the 

 phenomena without leading to new facts or pointing out 

 new lines of research. 



The earth, as is now well known, is an excellent con- 

 ductor for the most feeble currents of electricity, provided 

 the contact with it of the electrified body be sufficiently 

 broad. The aerial covering which surrounds it, is however 



