CHAPTER III. 

 Bacteria. 



THE more our knowledge of these micro-organisms becomes 

 enlarged, the more difficult it is to give a general defini- 

 tion -of them. They are known in all forms, from the 

 smallest specks or spheres to green, alga-like filaments ; 

 and they occur very nearly in all possible localities, under 

 the most various conditions, as the cause of putrefaction or 

 decay (Saprophytes), of diseases (pathogenic forms), and of 

 fermentation (zymogenic forms). 



The first knowledge of these forms was obtained by placing 

 small quantities of the different substances under the micro- 

 scope and examining them with high powers. In putrefying 

 meat very small spherical bodies were found, which clearly 

 multiplied by successive divisions ; in sour milk short, rod- 

 like bodies occurred : and yi putrefying vegetable matter 

 larger spherical bodies and long, fine, thread-like forms ; in 

 saliva, on the contrary, very fine, spirally-twisted threads 

 were found, etc. On this account it was convenient to 

 provisionally retain these /orms, and to describe them as 

 so many distinct species. Cohn especially has earned credit 

 in this respect, since to him is due the first systematic 

 classification of bacteria. 



We will first consider the various forms and individuals 

 somewhat more closely. As before stated, the bacteria in 

 their simplest form occur as spherical bodies of different sizes, 

 often so small that they can only just be seen even with the 

 strongest powers, and only give evidence of their existence 

 as organisms by their multiplication by division. They are 



