CONDITIONS AFFECTING BACTERIA. 21 



germicidal effect. Here, as with other factors, the results vary 

 very much with the species under observation, and a distinction 

 must be drawn between a mere cessation of growth and the con- 

 dition of actual death. Some bacteria, especially occurring on 

 the dead bodies of fresh fish, are phosphorescent. 



Conditions affecting the Movements of Bacteria. — In some 

 cases differences are observed in the behaviour of motile bacteria, 

 contemporaneous with changes in their life history. Thus, in 

 the case of bacillus subtilis, movement ceases when sporulation 

 is about to take place. On the other hand, in the bacillus of 

 symptomatic anthrax, movement continues while sporulation is 

 progressing. Under ordinary circumstances motile bacteria 

 appear not to be constantly moving but occasionally to rest. 

 In every case the movements become more active if the tem- 

 perature be raised. Most interest, however, attaches to the 

 fact that bacilli may be attracted to certain substances and 

 repelled by others. Schenk, for instance, observed that motile 

 bacteria were attracted to a warm point in a way which did not 

 occur when the bacteria were dead and therefore only subject 

 to physical conditions. Most important observations have been 

 made on the attraction and repulsion exercised on bacteria by 

 chemical agents, which have been denominated respectively 

 positive and negative chemiotaxis. Pfeffer investigated this 

 subject in many lowly organisms, including bacterium termo 

 and spirillum undula. The method was to fill with the agent 

 a fine capillary tube, closed at one end, to introduce it into a 

 drop of fluid containing the bacteria under a cover-glass, and 

 to watch the effect through the microscope. The general result 

 was to indicate that motile bacteria may be either attracted or 

 repelled by the fluid in the tube. The effect of a given fluid 

 differs in different organisms, and a fluid chemiotactic for one 

 organism may not act on another. Degree of concentration is 

 important, but the nature of the fluid is more so. Of inorganic 

 bodies salts of potassium are the most powerfully attracting 

 bodies, and in comparing organic bodies the important factor 

 is the molecular constitution. These observations have been 

 confirmed by Ali-Cohen, who found that while the vibrio of 

 cholera and the typhoid bacillus were scarcely attracted by 

 chloride of potassium, they were powerfully influenced by 

 potato juice. Further, the filtered products or the growth of 



