344 WRITINGS OF JOSEPH HENRY. [1855- 



rarified air than through that of ordinary density. The 

 light which accompanies a discharge in this case assumes 

 different colors, the violet predominating. This is a fact of 

 interest in connection with the color exhibited by lightning, 

 and we may infer that the discharges of a violet hue take 

 place between clouds at a great elevation in the atmosphere. 



The electric spark, when passed through a confined por- 

 tion of atmospheric air, is found to produce a chemical com- 

 bination of its component parts, namely nitrogen and 

 oxygen, and to form nitric acid. The same result is pro- 

 duced on a grand scale in the heavens during thunder- 

 storms; hence the rain water that falls, (in the summer 

 season especially,) always contains a considerable quantity of 

 nitric acid, which is considered by the chemist as furnishing 

 a portion of the nitrogen essential to the growth and devel- 

 opment of the plant : and to the same source is referred the 

 nitric acid in the nitrate of lime and potash found in the 

 form of efflorescence on damp ground and the walls of old 

 buildings. Indeed, all the nitrate of potash from which 

 gunpowder is manufactured is supposed to have its origin 

 in this way, and the explosion from the thunder-cloud 

 and that from the cannon, may be looked on as in one 

 sense — the counterparts of each other. 



Again, during the transmission of electricity from an or- 

 dinary electrical machine a pungent odor is perceived, some- 

 thing analogous to that produced by the slow combustion 

 of phosphorous, which Professor Schonbein, by a long- 

 continued series of researches, has shown to result from a 

 change in the oxygen of the air. He supposes that this sub- 

 stance is composed of two atoms, which by their combina- 

 tion partly neutralize each other, but which are separated 

 by the repulsion of the electric spark, and when thus set free 

 — have a much greater tendency to combine with other sub- 

 stances than in their ordinary state of union. Oxygen thus 

 changed or dissociated is called ozone, and as it would ap- 

 pear, performs an important part in many of the molecular 

 and chemical phenomena of the atmosphere. To this in- 

 creased combining power of oxygen ma}* be attributed the 



