20 GENERAL MORPHOLOGY AND BIOLOGY 



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 this 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 of the growth of 

 many bacteria have been found to have powerful chemiotactic 

 properties. It is evident that all these observations have a 

 most important bearing on the action of bacteria, though we do 

 not yet know their true significance. Corresponding chemio- 

 tactic phenomena are shown also by certain animal cells, e.g. 

 leucocytes, to which reference is made below. 



The Parts played by Bacteria in Nature. As has been said, 

 the chief effect of bacterial action in nature is to break up into 

 more simple combinations the complex molecules of the organic 

 substances which form the bodies of plants and animals, or 

 which are excreted by them. In some cases we kno*v some of 

 the stages of disintegration, but in most cases we know only 

 general principles and sometimes only results. In the case of 

 milk, for instance, we know that lactic acid is produced from 

 the lactose by the action of the bacillus acidi lactici and of 

 other bacteria, and that from urea ammonium carbonate 

 is produced by the micrococcus ureae. That the very compli- 

 cated process of putrefaction is due to bacteria is absolutely 

 proved, for any organic substance can be preserved indefinitely 

 from ordinary putrefaction by the adoption of some method of 

 killing all bacteria present in it, as will be afterwards described. 

 This statement, however, does not exclude the fact that molecular 



