CHEMISTRY OF A VOLCANO. 125 



tween July 1841 and June 1842. Twelve of 

 these occurred in one day in July. The shocks are 

 found to be most frequent in autumn and winter ; 

 and it has been noticed that very wet weather 

 not unfrequently precedes their occurrence. 



It is a very singular fact, that the instruments 

 called magnetometers employed in studying the 

 phenomena of terrestrial magnetism, indicate 

 with great delicacy also the occurrence of earth- 

 quakes. Those employed at Dublin indicated 

 from ten to twenty shocks in one year. 



Another evidence of subterranean movements 

 is the Volcano. Every stage of volcanic vio- 

 lence is attended with peculiar chemical phe- 

 nomena. At first, when the vast artillery of 

 nature opens fire, glowing ashes shoot up into 

 the heavens, then a molten flood of lava is 

 pressed up into the crater, and rolls down in 

 devastating terrors upon the smiling country 

 below. These are accompanied with the pouring 

 forth of clouds of steam, and occasionally electric 

 flashes dart across the lurid flames. At a 

 later period, steam, sulphuretted hydrogen and 

 carbonic acid gases, are the only symptoms of 

 chemical activity, and lastly, when the fire is 

 almost- extinct, carbonic acid gas alone rises 

 from the once fire-glowing crater. Vapours of 

 hydrochloric and sulphurous acids, together with 

 pure nitrogen gas and ammonia, have also been 



