82 DISIXFECTAXTS 



great difference between the results of our own experi- 

 ments and those by Chick and Martin, with HgCl2. 

 These authors measured the weakening effect of blood 

 serum not by comparison with water, but by comparison 

 with a solution of about 0.15% meat extract plus pep- 

 tone, which eliminated a good portion of the mercury 

 ions, while in our controls no protein was present except 

 that of bacterial cells. 



The efficiency of chlorine and iodine is greatly de- 

 creased by very small amounts of organic matter. For 

 city water supplies, 0.2 to 0.5 ppm. of chlorine is suffi- 

 cient to bring about rapid sterilization while 7 to 10 

 ppm. is the amount ordinarily used for sewage, which 

 contains only 600 ppm. of foreign matter, less than is 

 present in the medium used for the determination of the 

 phenol coefficient. 



The H and OH ions are very important foreign sub- 

 stances. Some disinfectants and antiseptics act very 

 differently at different pH values. In general, however, 

 with strong disinfectants, the effect of a change in pH 

 is not very great. There is only one strong disinfectant 

 which shows a very great response to changes in acidity, 

 namely chlorine. Some antiseptics are made absolutely 

 harmless by a change in pH. This is especially true with 

 such compounds as benzoic, salicylic, and sulfurous 

 acids, which are efficient only at low pH, and also with 

 the dyes, all of which will be discussed in detail later. 



V. INTERFERENCE OF ANTIDOTES WITH DISINFECTION 



Antidotes are not employed very often in man's strug- 

 gle with bacteria. When chemical compounds are used 

 to kill bacteria, either they can be removed by simple 

 rinsing (skin, instruments), or the sterilized materials 

 are discarded afterwards (excreta, dead animals). For 

 materials to be used after sterilization disinfection by 



