DISINFECTANTS 117 



or preferably plotted in a graph which tells what is to be done 

 every minute, e. g., 



1 2 3 4 5 G 7 8 9 10 11 12 13 14 15 



X — o — — 



x-o-o — o — o — o 



X o o o 



where x indicates inoculation and o transfer of a loopful to 

 new broth. 



Thus only the disinfectant concentration of 0.05% is inocu- 

 lated at the start, the inoculation of the 0.2% concentration 

 comes one minute later, and that of the 0.1% solution not un- 

 til 5 minutes later. It is usually possible to work out a sched- 

 ule by which a large number of dififerent determinations can 

 be fitted together so as to avoid long waiting between transfers. 



With an entirely unknown substance, a preliminary test 

 will save time. A spacing of the concentrations in decimal 

 multiples such as 1%, 0.1% and 0.01%, or 400 ppm., 40 ppm., 

 4 ppm., and a spacing of the times in multiples of their log- 

 arithms are advisable. A good range of preliminary testing 

 times is that which includes 1, 3, 10, and 30 minutes, since 

 their logarithms are about 0.5 apart (0; 0.48; 1.0; 1.48). If 

 the results of such a preliminary test are entered into a double 

 logarithmic graph, the slope of the resulting curve tells at once 

 the more desirable combinations of times and concentrations to 

 be used. If, for example, one concentration is strong enough 

 to kill all bacteria in 1 minute while one-tenth of that concen- 

 tration does not kill in 30 minutes, this indicates a high con- 

 centration exponent, which is a warning not to space the con- 

 centrations too widely in the final test; in this case, a range 

 of 1%, 1.5% and 2% may suffice. Phenol, which at 2% kills 

 in less than i/o minute and at 0.2% requires more than 10 hours, 

 and KMnO^, which at 100 ppm. kills in less than 1 minute and 

 at 10 ppm. requires 10-20 hours, are typical examples of disin- 

 fectants with very high exponents. A glance at Figure 23 will 

 help in deciding the most appropriate range of dilutions to 

 be used. 



In order to prove that the test culture has a normal sensi- 

 tivity, the author has always determined simultaneously its 

 death time in a 1% phenol solution. 



