148 ANTISEPTIC AND GERMICIDAL POWER 



CHAPTER X 



THE DETERMINATION OF ANTISEPTIC AND GERMICIDAL 



POWER 



The testing of the germicidal power of a given substance is 

 simple in principle, but in practice there are many possibilities 

 of error. 



It is obvious that arithmetical statements of germicidal 

 power are useless unless both the duration of action and the 

 nature of the material acted upon are specified. In other words, 

 the three factors strength of the solution, duration of action, 

 and nature of the material acted upon cannot be separated 

 from one another. To give but one illustration : if we ascertain 

 that a given strength of mercuric chloride will kill typhoid 

 bacilli in broth culture in half an hour, it by no means follows 

 and, indeed, would be distinctly untrue to state that such a 

 strength in the same time would be sufficient to render typhoid 

 faeces harmless as a possible factor in the spread of enteric 

 fever. 



Laboratory determinations, owing to the rigid control to 

 which they are susceptible, have great value, but unless their 

 limitations are recognized, deductions from them to practical 

 conditions, which perhaps differ considerably in their nature, 

 may be very erroneous. 



It is very difficult to define standard conditions for testing 

 purposes, because in practice disinfectants are employed as 

 germicides under a variety of conditions. 



Disinfection has been shown by Madsen and Nyman, and 

 also by H. Chick, to be a process exhibiting many analogies 

 to a chemical reaction, one reagent being represented by the 

 bacterium and the second by the disinfectant. When the dis- 

 infectant is present in considerable excess, the process proceeds 

 in accordance with a definite law, the number of living bacteria 

 per unit volume progressively and regularly decreasing with 

 increase of time in a logarithmic ratio. 



