1911.] 



Hie Influence of Ionised Air on Bacteria. 



285 



Petrie dishes in position containing the bacteria. The photographs of Plate 11 

 of B. asiaticce cholerce, fig. 16 ; B. typhosus, fig. 17 ; and B. coli communis, 

 fig. 18 ; in hydrogen, with the discharging point arranged as in fig. 19, 

 show that the sterilising influence of the negative discharge is somewhat 

 greater in hydrogen at atmospheric pressure than in nitrogen or, by com- 

 parison with the previous results, than in air, owing possibly to the greater 

 velocity and range of ions in hydrogen. It would, however, appear that 

 in these cases the nature of the gas is not of the first importance, and 

 that it is the presence of the electrical charge which is the chief inhibiting 

 cause. 



The criticism having been made that water- vapour from the surface of the 

 agar might give rise to the formation of hydrogen peroxide in the glow, trials 



Fig. 19. — Discharge j)oint in tube. 



were made with the exposure vessel arranged as in fig. 19. The wire to which 

 the discharging needle is soldered is led down a glass tube through which a 

 stream of the gas to be used is slowly passed. The needle is held in a central 

 position by a glass bead b, and the gas, ionised by the discharge, is blown 

 gently upon the agar surface sown with bacteria. The Petrie dish rests on a 

 metal plate connected to the positive pole. There is no possibility of water- 

 vapour reaching the needle point, and the arrangement is very convenient for 

 the examination of the effect of ionised gases on surface cultures. In 

 operation the outlet from the jar was first sealed with paraffin wax and the 



