760 A MANUAL OF BACTERIOLOGY 



a considerable influence on the extent or rate of disinfec- 

 tion. Up to the optimum temperature at which the 

 organism to be disinfected grows in the medium in which 

 it is being exposed, the activity of a disinfectant may 

 decrease as the temperature rises, owing to increased 

 vigour of the organism. A relatively small difference 

 of temperature two or three degrees may make an 

 appreciable difference in the activity of the disinfectant, 

 and in the examination of disinfectants the failure to 

 remember this fact has led to serious error. A rise of 

 temperature above the optimum for growth increases the 

 activity of the disinfectant, sometimes to an enormous 

 extent. The same is sometimes the case, even at tempera- 

 tures below the optimum, when the organism is in un- 

 favourable conditions for growth. A mixture of dis- 

 infectants in many cases has a more powerful effect than 

 can be produced by either separately (Chamberland). 

 The resistance of bacteria to disinfection by chemical 

 agencies is extremely variable and is also selective. 

 Bacteria of one class may be many times more sensitive 

 to one disinfectant than to another when both substances 

 exert an equal effect on bacteria of another class. The 

 presence of organic matter may profoundly modify the 

 action of chemical disinfectants, particularly those acting 

 by oxidation, considerably reducing their efficiency. 



Requirements for an efficient disinfectant. The conditions 

 which should be satisfied by an efficient disinfectant for 

 general use are simple, but not easy to obtain. Because 

 a disinfectant effect depends on the strength of the solu- 

 tion, the substance should have an approximately definite 

 efficiency for particular organisms in given conditions. 

 In practice disinfectants must be used with water or in 

 an aqueous solvent ; it should, therefore, yield a stable 

 solution or uniform emulsion with water in all proportions. 

 Because bacteria as presented for practical disinfection 



