378 PUBLIC HEALTH BACTERIOLOGY 



acid. When used simply in solution, oxygen is liberated 

 in the nascent state, and oxidizes any organic matter 

 present (CaOCl 2 + H 2 O = CaO + 2HC1 + O). Peroxide 

 of hydrogen and potassium ' permanganate act similarly 

 as oxidizers. 



Carbolic acid (phenol) and the cresols (lysol and creolin) 

 do not dissociate, and their efficiency is increased by the 

 addition of NaCl and diminished by alcohol. Formalde- 

 hyde is not helped by adding salt, but alcohol is harmful. 



Standardization. The efficiency of any disinfectant 

 depends on many factors, namely, the strength used, the 

 solvent, the temperature, the bacterium to which applied, 

 the time allowed for action, the other substances present, 

 the number of bacteria present. For practical purposes 

 the strengths are expressed as percentages ; but in com- 

 parisons it is more scientific to work with solutions of 

 the molecular weight or multiples thereof in one litre. 



Coefficient of Inhibition. This term is applied 

 to the strength (of a chemical substance) which is able to 

 prevent the growth of a micro-organism ; that is, its 

 antiseptic value. 



It is determined by making broths or other media 

 containing the chemical substance to be tested, in a range 

 of strengths. Equal quantities of the bacterium used in 

 the test are inoculated into the various tubes (say, one 

 loopful), the contents mixed, and incubated. In the 

 case of solid media, they are poured into plates and the 

 colonies (if any) counted. In broth cultures, look for 

 turbidity, and confirm positive or negative results by 

 making films. The coefficient is expressed in terms of 

 strength and bacterium used. Thus, carbolic acid is 

 said to inhibit the growth of anthrax bacilli when present 

 in a strength of I part in 800 of medium. B. typhosus 

 requires 1-400, and Sp. cholerae, 1-600. Of corrosive 

 sublimate, 1-100,000 inhibits B. anthracis, and 1-60,000 

 inhibits B. typhosus. 



Germicidal, Bactericidal, or Disinfectant Strengths. 

 Koch used anthrax spores dried on silk threads, which 

 he immersed in various strengths of substance being 

 tested, at a definite temperature and for varying times. 



