164 PHARMACEUTICAL BACTERIOLOGY. 



destroying powers, but it must also be practically usable and it must not be 

 misrepresented as to its value and its application and use in practice. 



The resistance of pathogenic germs to disinfectants is extremely variable. 

 Furthermore, the various disinfectants produce changes in the tissues and 

 substances in and upon which they act, which changes tend to modify, check 

 or inhibit the disinfecting powers. Thus a number of disinfectants may 

 have the same laboratory phenol coefficient and yet their value as disinfec- 

 tants in actual practice is widely different because of the difference in the 

 effects produced in and upon the substances with which they are brought in 

 contact. 



As a rule, the action and use of disinfectants is variable according to the 

 following conditions: 



1. Disinfectants are more active when warm or hot. In all disinfections 

 hot solutions should be used, if possible and if practicable. 



2. Gaseous disinfectants act only in the presence of moisture, as will be 

 explained under formalin and sulphur disinfection. 



3. The thoroughness of disinfection is directly proportional to the time 

 that the disinfectants are allowed to act. 



4. The activity of disinfectants is directly proportional to the degree of 

 concentration, though there are noteworthy exceptions. Absolute alcohol, 

 for example, is of very little value as a disinfectant, whereas the weaker 

 solutions (40 to 70 per cent.) are a very active germ destroyer. The same is 

 true of ether, chloroform, glycerin and a number of other substances. Most 

 disinfectants have a concentration of optimum or maximum efficiency which 

 is the degree of concentration generally employed in practice. 



5. In actual practice the cost of disinfectants is a very important de- 

 sideratum, as is indicated by the following table giving the comparative 

 phenol coefficient and the relative cost: 



