110 DLSIXFECTAXTkS 



different concentrations instead of its relative value in 

 comparison with phenol, and also to measure the effect of 

 organic matter. Such a method requires more time, but 

 the far more useful result makes this expenditure of time 

 worth while. The method consists, in principle, in deter- 

 mining the time in which the standard bacterial suspen- 

 sion will be sterilized by various dilutions of the disin- 

 fectant. 



In the method of the phenol coefficient, the effect of 

 organic matter is not sufficiently eliminated, since 10% of 

 the suspension medium is broth. In the present method, 

 the bacteria are grown in broth in a centrifuge tube 

 which is centrifuged when the culture is 24 hours old, 

 and the cells are then re-suspended in sterile w^ater or in 

 a mineral solution known to prolong their life without 

 providing food. (Such solutions have been worked out 

 by Zeug, 1920.) The cell concentration is the same as it 

 was in the culture. 



Cultivation of the test organism and all other details of 

 the procedure are the same as for the determination of 

 the phenol coefficient. The disinfectant is made up in va- 

 rious concentrations (the pharmacologists will probably 

 continue to prefer "dilutions" to concentrations) and the 

 death time is determined as usual by transfer with a 

 standard loop into broth tubes. The culture may also be 

 tested against phenol so as to check unusual fluctuations 

 of the test organism. 



The results can be arranged in the form of a chart as 

 in Table 22 or in the form of a graph in which the death 

 times are plotted against concentrations on a double loga- 

 rithmic scale, as in Figure 23. The tabular form is handy 

 for the recording of data for further reference, but the 

 graph is far more instructive. Since the death time is 

 not known precisely, but lies within a certain range, the 

 entire range should be plotted on the graph as a line, and 

 not mer9ly its average (see Figures 17 and 23). Thus in- 

 stead of drawing the disinfection curve through points, 



