36 LOaARITBM/V ORDER OF DEATH 



logarithmic. This explanation to which one may object 

 again that it is entirely theoretical and not based upon 

 any measurements of membrane thickness (see however 

 Figure 12) — might explain chemical disinfection, but 

 it does not seem plausible for heat sterilization, and can 

 not explain the order of death in the case of X-rays 

 which penetrate with lightning speed. 



Lee and Gilbert (1918) call attention to the fact that 

 biological characteristics are as a rule distributed in a 

 manner quite different from that which has been as- 

 sumed in the various theories of graded resistance. "In 

 view of these observations, the authors are led to the 

 conclusion that the logarithmic nature of the disinfec- 

 tion process is due to a general similarity of the indi- 

 viduals in a given pure culture rather than to a dissim- 

 ilarity of the individuals." 



The third group of explanations is based on the analogy 

 with monomolecular chemical reactions. The all-inclusive 

 statement by Paul (1909) that bacteria are so small that 

 they react chemically like large colloidal molecules 

 will probably not be taken seriously by the biologists of 

 today. 



More in line with present conceptions is the explana- 

 tion proposed by Isaacs (1935) who elaborated on the 

 contention of many biologists that enzyme inactivation 

 is probably the first step in the death of the cell. The 

 fact that enzyme inactivation is frequently a monomole- 

 cular process led Isaacs to the hasty conclusion that the 

 logarithmic order of enzyme inactivation is the evident 

 cause of the logarithmic order of death of bacteria. 



This conclusion is based on a wrong application of the 

 law of mass action. When an enzyme solution, distrib- 

 uted in 100 test tubes, is heated, and one of these tubes 

 shows 90% of inactivated molecules, we are certain that 

 all other test tubes at that moment also contain 90% of 

 inactivated molecules. The order of enzyme inactiva- 

 tion in each test tube is logarithmic, and the reaction 



