Fog Formation 111 



FOG FORMATION IN AIR WHICH HAS PASSED 

 THROUGH A SILENT DISCHARGE. 



By F. O. Anderegg and K. B. McEachron, Purdue University. 



The corona discharge has been studied in the Chemical Laboi'a- 

 iory^, ■ and Electrical Engineering Experiment Station', ' of Purdue Uni- 

 versity M^ith especial reference to the formation of nitric acid and ozone 

 in air. One of the more interesting of the phenomena observed while 

 working under a large variety of conditions has been the formation of 

 fog in the air passed first through a discharge and then through a 

 solution for the absorption of the nitric acid anhydride or the ozone 

 formed in the discharge. An important connection between fog forma- 

 tion and the yield of nitric acid was suspected. This led to a study of 

 fog formation in the corona work during a period of several years, the 

 results of which are recorded here. 



A fog, in the first place, may be defined as the suspension of finely 

 divided particles of liquid, (or frozen liquid if the temperature is low 

 enough), in a gas. In order to have a fog there must be moisture in 

 the atmosphere, usually approaching the saturation point. There must 

 be also nuclei on which the fog may be condensed because it has been 

 shown that in the absence of all nuclei a large supersaturation is re- 

 quired to condense the water'. Nuclei may be dust particles, ions'' or 

 chemical substances of a hygroscopic character''. The absence of dust 

 particles sufficient to cause formation under the conditions used is shown 

 by the fact that during the sweeping out of the apparatus preliminary 

 to discharge no fog was observed at any time. Ions did not serve as 

 nuclei for the following reasons: C. T. R. Wilson'' shows that a cer- 

 tain amount of supersaturation is necessary before water vapor will 

 condense on ions. Passing the air coming out of the discharge tube 

 through a large electrostatic field failed to have any effect on the fog 

 either before or after the discharge was turned on. 



There remain as nuclei only hygroscopic compounds. That it was 

 due to the chemical substances was proved by dividing the stream of 

 air after leaving the discharge. One-half passed directly into the ab- 

 sorption system and fog was observed as usual. The remainder of the 

 gas passed through a large tube containing asbestos impregnated with 

 silver so that the ozone was decomposed completely and the nitrogen 

 pentoxide reacted with the silver oxide so formed to produce silver 

 nitrate". The air then passed through an absorption apparatus and 

 gave no evidence of either ozone or oxides of nitrogen or fog. Ozone 

 alone in contact with moisture has a slight tendency to produce ioniza- 



1 Anderess. J. Am. Chem. Soc. 30. 2581 (1917). 



= Ray and AndeiesK. Ibid. 43, 967 (1921). 



3 Harding and McEachron. J. Am. Inst. Elec. Eng. April. 1920. 



■* McEachron and George. Purdue Univ. Eng. Exp. Sta. Bull. 9. 



= C. T. R. Wilson. Phil. Trans. 189A, 265 (1897) ; 193A. 289 (1899). 



« Bancroft. J. Phys. Chem. 22, 312 (1918). 



"Proc. 38th Meeting, 1922 (1923)." 



