DISINFECTANTS 115 



that of other disinfectants. (As was said above, this ac- 

 counts for the great discrepancies in the values of the 

 phenol coefficient determined by different workers for a 

 given disinfectant.) 



The disinfection curves for different temperatures 

 will be parallel lines equidistant for equal temperature 

 changes as long as the temperature coefficient is constant. 



The efficiency of a few disinfectants, such as chlorine 

 and mercurochrome, varies greatly with the acidity of 

 the test solution. This effect can be measured by draw- 

 ing separate disinfection curves for each pH that is of 

 interest. The lines will be jDarallel, as a rule, and their 

 distance measures the effect of pH on the disinfecting 

 power. 



Justification of the N e iv Features 

 of the Method. The advisability of measuring 

 the death times for several different concentrations is 

 quite evident from the different slopes of the disinfec- 

 tion curves of the various disinfectants. These different 

 slopes indicate how differently the efficiency of the disin- 

 fectants is affected by changes in concentration. 



For the determination of the effect of organic matter, 

 skimmed milk seems ideal because its composition is re- 

 liably uniform all over the world, and because it contains 

 dissolved proteins and carbohydrates as well as finely 

 suspended organic matter (casein). 



The resuspension of centrifuged bacteria in water is 

 necessary to remove the broth which contains sufficient 

 organic matter to interfere with such strong disinfect- 

 ants as HgCL or iodine. In the F.D.A. method for the 

 phenol coefficient, the Vii cc. of culture used brings 1500 

 ppm. of organic matter into the disinfectant. One cen- 

 trifugation reduces it to about 25 ppm. dissolved organic 

 matter besides 20 ppm. of bacterial solids. A second 

 washing (which is advisable with such disinfectants as 

 chlorine and iodine) reduces it to about 0.5 i^pm. 



