82 BACTERIA 



especially in the case of animal infusions, quickly becomes 

 very strong and disagreeable. 



The scum after attaining a perceptible thickness breaks up 

 and falls to the bottom, and after this the fluid slowly clears 

 again, becoming once more quite transparent and losing its 

 bad smell. If exposed to the light patches of green appear 

 in it sooner or later, due to the presence of microscopic 

 organisms containing chlorophyll. The fluid has acquired, 

 in fact, the characteristics of an ordinary stagnant pond, and 

 is quite incapable of further putrefaction. The whole series 

 of changes may occupy many months. 



Microscopic examination shows that the freshly-prepared 



I 

 f 



t 



FIG. 14. Bacterium termo. A, motile stage : B, vesting stage or 

 zooglsea. (From Klein.) 



fluid is free from organisms, and indeed, if properly filtered, 

 from particles of any sort. But the case is very different 

 when a drop of infusion in which turbidity has set in is 

 placed under a high power. The fluid is then seen to be 

 crowded with incalculable millions of minute specks, only 

 just visible under a power of 300 or 400 diameters, and all 

 in active movement. These specks are Bacteria, or as 

 they are sometimes called, microbes or micro-organisms ; 

 they belong to the particular genus and species called 

 Bacterium termo. 



Seen under the high power of an ordinary student's 

 microscope Bacterium termo has the appearance shown in 



