130 ANTISEPTICS 



slow growth was observed with 0.006% formaldehyde, 

 but none with 0.008%. The phenol efficiency was slight- 

 ly altered by acidity ; at pH 5.3, complete inhibition was 

 obtained by 0.12%, while at pH 3.1, slow growth was 

 observed with 0.14%. 



The effect of phenol consisted largely in a decrease of 

 the rate of multiplication (Fig. 24). There was no in- 

 crease in the lag phase. The generation times, computed 

 from plate counts, are represented in Table 25. Direct 

 microscopic counts of dead cells were also made after 

 mixing 1 cc. of the phenol-treated cultures with 1 cc. of 

 of 0.01% methylene blue solution. The proportion of 

 dead cells never exceeded 5%. This observation per- 

 mits the conclusion that the increased generation time 

 was not the result of excessive death in a population 

 which otherwise was multiplying normally, but was due 

 to a lower average growth rate. The final number of 

 cells capable of growing with or without phenol was de- 

 termined only roughly, but unquestionably it was small- 

 er in the presence of the disinfectant. 



Formaldehyde produced the same symptoms of de- 

 creased rate of multiplication and decreased maximal 

 jDopulation, but in addition it caused a quite extended 

 lag (Table 25 and Figure 24). 



As far as the observations of the writer go, formal- 

 dehyde is the only strong disinfectant which causes such 

 a marked prolongation of the lag period. Mercurochrome 

 also increases the lag, but to a much lesser extent. The 

 inhibition of growth by formaldehyde is so prolonged 

 that in determining the phenol coefficient of this disin- 

 fectant, the usual observation time of 48 hours gives 

 wrong results and nnist be extended to 4 days. 



These two diluted disinfectants are alike in one re- 

 spect: as long as the dose is sufficient to prevent growth, 

 they kill the cells slowly but continuously. With weaker 

 doses, a slow growth sets in, but many cells die before 

 multiplication starts. The rate of multiplication de- 



