82 CHEMOTAXIS AND CHEMICAL DISINFECTION 



circumstances, when temperature, food, and other conditions are all 

 optimal. Bacteria are, in short, like all other organisms, strongest and 

 most resistant when they are in the best health. There is always a 

 great difference between spores and the far less resistant sporeless vege- 

 tative cells, so that a disinfectant can only be considered effective when it is 

 able to kill spores. In practice, circumstances may sometimes arise when 

 this requirement may be dispensed with. 



Every disinfectant must be tested on the following three points : 



I. In what concentration must it be added to a given substratum in 

 order to prevent the development and multiplication of bacteria without 

 killing them ? This is the coefficient of inhibition. 



II. What is the shortest time in which sporeless bacterial cells in water 

 at ao°-25° C. are killed by a substance in medium concentration (i. e. not 

 sufficient to cause plasmolysis or other injury) ? This is the inferior lethal 

 coefficient. 



III. What is the shortest time in which, under the same conditions, the 

 spores are killed ? This is the superior lethal coefficient. 



Laborious investigations without number have been made with the 

 object of determining these three values for all kinds of organic and 

 inorganic compounds, and we have now a number of carefully-chosen 

 substances that are suitable for use as disinfectants. The following tables 

 give a few examples. Further details will be found in the works cited 

 in Notes 3 and 54. 



I. Coefficient of Inhibition for Anthrax Bacilli in 

 Ox-blood Serum. From Behring (54). 



The numbers indicate the number of cubic centimetres of serum in 

 which one gram of the solids or one cubic centimetre of the liquids was 

 dissolved ; for instance, ' Corrosive Sublimate, 10,000 ' = 1 gram HgCl 2 in 

 10,000 c.c. serum : 



Cyanin and malachite green 40,000 



Nitrate of silver 30,000 



Corrosive sublimate 10,000 



Trichloride of iodine i>5°o 



Caustic soda 1,500 



Cadaverin (bacterial toxine) IjSoo 



Quinine hydrochlorate 500 



Carbolic acid 500 



Thymol 250 



Salicylate of soda 1 50 



Alcohol 15 



Common salt 15 



The extraordinarily small quantities which are in some cases sufficient to 



