118 DISINFECTION 



Viability of Bad. coli in Distilled Water after 9 Hours at 37° C. * 



* pH was adjusted by minute additions of acid or alkali. 



t This figure suggests that a slight increase in the numbers of surviving organisms may have 

 occurred. 



The maximum viability occurs at pH 6-0. Colien (1922) has likewise shown 

 that when the pH of water is stabilized by the addition of buffer salts, the results 

 are much more regular. 



Other factors that may influence the action of distilled water on bacterial 

 viability are traces of alkali absorbed from the glass, the amount of COg absorbed 

 from the air, the quantity of dissolved oxygen, and the temperature at which the 

 suspension is maintained. Whipple and Mayer (1906), studying the length of 

 life of Sahn. typhi in sterile tap water, showed that it remained viable for nearly 

 2 months when the water was exposed to the air, but died out in 4 days 

 when the water was kept under anaerobic conditions. Houston (1914), likewise 

 working with Salm. typhi, found that, when suspended in water kept at 0° C, 

 it lived for 8 weeks, at 18° C. for 3 weeks, and at 37° C. for only 1 week. 



Summing up, we may say that the length of life of vegetative bacteria in dis- 

 tilled water is influenced by a large number of factors. When these factors are 

 all favourable, the organisms may remain viable for considerable periods ; when 

 unfavourable, they may die out in a very short time ; further, the eSect varies 

 greatly with difierent species of organisms. There is no evidence that distilled 

 water acts by causing disruption of bacteria, as it does of many unicellular 

 organisms ; bacteria are too resistant to changes in osmotic pressure for this to 

 be probable. 



Acids. — Kronig and Paul (1897) were the first to show that the disinfectant 

 action of acids in general is proportional to their degree of electrolytic dissociation, 

 i.e. to the H-ion concentration of their solutions. Some figures of Winslow and 

 Lochridge (1906) will make this clear. Comparing the strengths of HCl and 

 H2SO4 necessary to produce a 99 per cent, and a 100 per cent, reduction in the 

 numbers of Bact. coli in 40 minutes, they observed that the disinfectant action 

 of these two acids was in proportion to their degree of dissociation. Their results 

 are given in Table 5. 



TABLE 5 



Showing Percentage Reduction of Bact. coli in 40 Minutes by Acids of Different 



Strengths. 



