INTRODUCTION. 



IN the outer world with which mankind is in daily con- Significance 

 tact, and which forms the subject of hygienic research, organisms. 

 the attentive observer finds micro-organisms widely dis- 

 tributed and only just visible with the aid of the best 

 optical means; these bodies, however, on account of 

 their silent yet widespread and energetic activity, play 

 an important part in the economy of nature and in the 

 existence of mankind. They occasion the destruction 

 of dead organic material, they cause the oxydation of 

 substances otherwise very resistant, and constantly pro- 

 vide new nutriment for chlorophyllous plants ; they set 

 up the most varied fermentations, and are indispensable 

 aids in the preparation of our ordinary nutritious and 

 savoury food ; on the other hand they attack our plants 

 as parasites, bringing degeneration and death to their 

 hosts ; at times they cause the severest diseases in the 

 lower and higher animals, and threaten even man with 

 fatal epidemics. In no department of hygiene is their 

 influence missed ; in the air, in the water, in the soil, 

 in our immediate surroundings, in our dwelling, and 

 in our food, the same minute organisms are present 

 as constant companions, and at times as dangerous 

 foes. 



The majority of these important yet minute bodies Synonyms. 

 are plants of the most elementary structure and with 

 the most simple mode of propagation, but yet with 

 extraordinary powers of multiplication. They are 

 grouped together under the headings "micro-organisms " 

 or "microbes"; at times they are designated a^ 

 "lower fungi" or as "bacteria." Further, various 

 terms are applied to particular groups of these organ- 

 isms according to the particular mode of action to 

 which attention is directed. Thus, from the physio- 



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