284 



Prof. W. M. Thornton. 



[June 20, 



The two upper photographs, fig. 13 (Plate 10) are ot B. coli communis, the 

 right hand exposed for 1 hour, the left for \ hour, in the latter of which the 

 signs of clearing are slight. 



The organism most sensitive to the point discharge of any so far examined 

 is B. asiaticce choleras, two exposures of which are given in fig. 14 (Plate 10), 

 that to the left having been exposed for a quarter of an hour, that to the 

 right for half an hour. This bacterium is also sensitive to light, dying out 

 in a few days in diffused daylight. 



In the case of B. dysenterica Shiga, fig. 15 (Plate 10), half an hour had 

 little or no effect, the whole surface being covered with fine densely packed 

 colonies. In an hour, however, most of the plate was cleared. This and 

 the above result with B. asiaticce choleras may be compared with the observa- 

 tion of Buchner, that the growth of B. typhosus is prevented by exposing 

 a freshly-sown culture for an hour to full sunlight. 



From the above results it may be concluded that: (1) air ionised by either 

 positive or negative point discharge has a strong bactericidal action ; (2) the 

 negative discharge is much more effective' than the positive for short 

 exposures, though the result after many hours' exposure is nearly the same for 

 both. It is possible that some part of the bactericidal action of the positive 

 wind is owing to negative ions produced at the positive point. 



7. Since oxygen is electro-negative and ozone is known to be produced by 

 electrical discharge, the electric wind was examined for ozone by paper which 

 had been moistened with a solution in alcohol of tetra-methyl-^-p.-diamido- 

 diphenyl-methane.* This has the property of turning violet in the presence 

 ozone, yellow with nitric oxide. On exposure to the discharge arranged as in 

 fig. 2. it was found that ozone was produced at the negative point and to a 

 much less degree at the positive. There was no yellow coloration, though 

 the paper dried a purplish brown. The presence of ozone in the wind 

 suggested the possibility that the bactericidal action might only be indirectly 

 the result of ionisation, that the well-known sterilising influence of ozone 

 (whatever the ultimate cause of that may be) might explain the facts equally 

 well. To decide this, exposures were made in nitrogen and pure hydrogen- 

 The former was, however, found to contain - 8 per cent, of oxygen, and since 

 nitrogen when mixed with oxygen even in small quantities cannot be regarded 

 as quite inert, the result given in fig. 20, obtained in it using B. typhosus, 

 is chiefly of interest in showing that the effect is of the same order of 

 magnitude as in hydrogen and air. In the case of hydrogen the bell-jars were 

 well exhausted and filled with the gas several times before exposure, with the 



* See Fischer and Braemar, "On the Production of Ozone by Ultra- violet Light," 

 ' Ber., : 1905, vol. 38, No. 3, p. 2633. 



