98 BIOLOGY AND TECHNIQUE 



The numbers of bacteria exposed to the disinfectant, further- 

 more, is a factor which should be kept constant in comparative 

 tests. The medium, moreover, in which bacteria are brought into 

 contact with the disinfectant is a matter of great importance, inas- 

 much as either by entering into chemical combination with the dis- 

 infectant it may detract from its concentration or by coagulation 

 it may form a purely mechanical protection for the microorganism. 

 Thus bacteria which may be destroyed in distilled water or salt- 

 solution emulsion with comparative ease, may evince an apparently 

 higher resistance if acted upon in the presence of blood serum, 

 mucus, or other albuminous substances. Temperature influences 

 bactericidal processes in that most chemical disinfectants are more 

 actively bactericidal at higher than at lower temperatures, a fact 

 due most likely to the favorable influence of temperature upon all 

 chemical reactions. 53 As far as merely inhibitory or antiseptic 

 values are concerned, however, the temperature least favorable for 

 the reaction of the antiseptic is that which represents the optimum 

 growth temperature for the microorganism in question and the in- 

 hibitory effects of any substance are less marked at this point than 

 at temperatures above or below it. 



The important influence exerted by the solvent in which the 

 disinfectant is employed has already been discussed. For ordinary 

 work it is customary to express absolute and comparative antiseptic 

 and bactericidal values in terms of percentages based upon weight, 

 and this, beyond question, is both simple and practical. For strictly 

 scientific comparisons, however, as Kronig and Paul 54 have pointed 

 out, it is by far more accurate to work with equimolecular solutions. 



Eideal and Walker 55 have devised a method of testing disin- 

 fectants, in which an attempt is made to' establish a standard for 

 comparisons. They choose, as the standard, carbolic acid, and es- 

 tablish what they call the "carbolic-acid coefficient," This coeffi- 

 cient they obtain in the following way: the particular dilution of 

 the disinfectant under investigation which will kill in a given time, 

 is divided by the strength of carbolic acid which, under the same 

 conditions, will kill the same bacteria in the same time. We quote 



63 v. Behring, "Bekaempf. cler Infektions-Krankh., Infektion u. Desinf ection, 

 Leipzig, 1894. 



64 Kronig und Paul, loc. cit. 



65 Eideal and Walker, Jour, of the Sanitary Ins., London, xxiv. 



