CHAPTER III. 



EFFECT OF ANTISEPTICS AND DISINFECTANTS ON BACTERIA. 



In the previous cliapter several conditions were alluded to which 

 affected the growth of bacteria, such as the nature of the 

 nutrient soil, temperature, light, and electricity. The effect of 

 certain chemical substances, and of excessive heat and cold, was 

 also mentioned ; but this constitutes a subject of such practical 

 importance that it must be considered more fully. 



Agents which retard the growth of bacteria are generally spoken 

 of as antiseptics, as distinguished from disinfectants which altogether 

 destroy their vitality. 



Though chemical disinfectants, or germicides, when diluted, act 

 as efficient antiseptics, the converse, that an antiseptic in a suffi- 

 ciently concentrated form will act as a disinfectant, is not the case. 

 The term " antiseptic," indeed, should be restricted to those sub- 

 stances or agents which arrest the changes bacteria produce, but 

 which do not prevent their springing into activity when removed 

 to favourable conditions. Thus excessive heat, which destroys 

 bacteria and their spores, is a true disinfectant ; and excessive cold, 

 which only benumbs them, retarding their development without 

 killing them, is an antiseptic. 



Spores have a greater power of resisting the action of these 

 various agents than the parent cells, and many species of micro- 

 organisms differ from each other in their resisting power. An 

 exact knowledge of the subject can, therefore, only be based upon 

 investigations which will determine the effect of these agents upon 

 pure cultivations of the different micro-organisms causally related to 

 putrefaction and disease. In the latter case, especially, this is not 

 possible in the present state of our knowledge. In some cases of 

 communicable disease there is considerable doubt as to the etiological 

 importance of the organisms which have been described ; in other 

 cases no organisms have as yet been discovered, or the organisms 



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