LIQUID DISINFECTANTS 149 



Practical Application of Germicides. — We shall not give more than a brief 

 account of the use of germicides in practice, and shall confine ourselves to general 

 remarks, illustrating the application of the principles that we have already con- 

 sidered. 



Gaseous Disinfectants. — ^The gaseous disinfectants most commonly employed 

 are sulphur dioxide, chlorine, and formaldehyde. The first two are active only 

 in a moist atmosphere. Thus sulphur dioxide combines with water to form sul- 

 phurous acid, and chlorine to form hypochlorous acid. To be successful in the 

 destruction of vegetative bacteria, sulphur dioxide should be present in a con- 

 centration of 2-3 per cent., chlorine of 1 per cent., and formaldehyde 1-2 per cent, 

 of the atmosphere. Ozone is sometimes used for the sterilization of water and for 

 meat preservation. Heise (1917) found that concentrations by volume of about 

 1/1000 destroyed 95 per cent, of coliform organisms on the surface of an agar 

 plate in 1 minute, concentrations of about 1/270,000 in 1 hour, and concentrations 

 of about 1/720,000 in 3-4 hours. The gas has little penetrating power, and is 

 of value only for the destruction of organisms unprotected by colloidal or other 

 material (see Elford and van den Ende 1942). The use of aerosols and vapourized 

 disinfectants for sterilization of the air — as apart from fumigation — is considered 

 in Chapter 91. 



Liquid Disinfectants. — In actual practice it is inevitable that disinfectants 

 should be employed more or less empirically ; it is impossible, from knowledge 

 gained in the laboratory, to predict exactly the length of time requisite for the 

 complete sterilization of any material. Realizing this, we err on the safe side, 

 and arrange our conditions so as to obtain Sterilization in a time much shorter 

 than that which is actually allowed. To do this, however, it is necessary to take 

 into consideration the principles that we have already considered, so far as they 

 are known, and pay particular attention to such variables as the nature of the 

 organism, the material in which it is contained, the H-ion concentration, the salt 

 content, and the temperature at which the reaction is to proceed. Having con- 

 sidered these, the disinfectant to be chosen, the concentration in which it shall 

 be allowed to act, and the time for which its action shall continue, may be deter- 

 mined. For general purposes, we may lay down a few simple rules. 



(1) Spores are more resistant than vegetative bacteria. 



(2) Bacteria possessing a high content of lipins, such as the acid-fast bacilli, 

 are very resistant to liquid disinfectants. Tubercle bacilli in sputum may with- 

 stand 5 per cent, phenol for 24 hours, but they are killed by boiling in 1 minute. 



(3) For the destruction of spores and acid-fast bacilli heat is preferable to 

 chemical disinfectants. 



(4) Bacteria suspended in a protein medium are more resistant than those in 

 a non-protein medium. 



(5) If the protein medium is also a good nutrient medium, the organisms are 

 even more resistant. 



(6) Disinfection by nearly all germicides proceeds more quickly in an acid 

 than in an alkaline medium. There is evidence that a given concentration of 

 H-ions is more bactericidal than of OH-ions. 



(7) The effect of salts in the medium depends on their nature and on their con- 

 centration. In general, salts increase the action of phenol and of the emulsified 

 disinfectants, but diminish that of HgClj. 



