CHAPTER XXI. 



ANTISEPTICS. 



IN former chapters we. have on several occasions men- 

 tioned that a variety of substances and conditions are 

 capable of exerting a detrimental influence on the life and 

 growth of micro-organisms. Amongst these are The 

 presence of certain substances in the nutrient soil, the 

 temperature, and some chemical products, such as those 

 belonging to the aromatic series, phenol, indol, skatol, &c. 

 The presence of certain substances in the nourishing 

 material is, as we have seen, an essential condition, cceteris 

 paribus, for the growth and multiplication of micro-organisms. 

 Thus pathogenic organisms do not thrive in an acid medium, 

 they cannot thrive if proteids or allied compounds and cer- 

 tain inorganic salts are absent : putrefactive and zymogenic 

 organisms, on the other hand, or, at any rate, some of them, 

 are capable of thriving well in acid media (e.g. the bacillus 

 subtilis in acid hay-infusion, the micrococcus ureae in acid 

 urine). Further, many (not all) pathogenic organisms can- 

 not thrive unless they are exposed to a certain degree of 

 warmth ; they thrive best at blood-heat, while putrefactive 

 and many zymogenic organisms thrive well at ordinary 

 temperatures, though of course their growth is more rapid 



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