112 ATLAS OF BACTERIOLOGY. 



ones). It is sometimes very slow, sometimes so 

 rapid that observations in detail are hardly possible 

 (bacterium typhi). 



In some cases it is difficult to decide whether there 

 is a real active movement or whether the micro-organ- 

 isms do not exhibit an unusual degree of the so- 

 called Brownian molecular movement i.e., the danc- 

 ing and trembling which are also found in finely 

 divided, non-organized particles. In such cases, 

 apart from the attempt to render the flagella visible, 

 it is well to examine the organism in a drop of five- 

 per-cent carbolic acid or one-per-cent corrosive subli- 

 mate. If the movements then continue, we have had 

 to deal only with molecular movements. Some 

 varieties do not always exhibit movements of their 

 own, but they may be absent in certain nutrient 

 media. According to A. Fischer the vital movements 

 may be lacking, although the flagella are perfectly 

 developed for example, in bacillus subtilis upon a 

 nutrient medium containing two to four per cent 

 ammonium chloride. In two different cultures of 

 micrococcus agilis Ali-Cohen, drawn from a good 

 source, we saw neither vital movements nor flagella, 

 and reached the conclusion that the same variety 

 may occur with or without flagella. 



Certain chemical substances attract bacteria (posi- 

 tive chemotaxis), others repel them (negative chemo- 

 taxis). Oxygen in particular attracts aerobic, and 

 repels anaerobic bacteria. As Beyerinck showed, 

 very beautiful chemotaxic or aerotaxic figures can be 

 obtained in the following way : In a test tube filled 

 three-quarters full with sterilized water is placed an 

 unsterilized bean, pea, or the like. By diffusion the 



