26 GENERAL MORPHOLOGY AND BIOLOGY. 



surface of a cover-glass. On the outer end of the rod being 

 warmed, heat waves were, of course, conducted up to the 

 point and the bacteria swarmed round the latter. 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. Pfeiffer 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. Fallacies due 

 to the passing of the fluid out of the tube otherwise than 

 by diffusion, to temperature changes, and to vibration, 

 seem to have been excluded, and control experiments were 

 performed with dead bacteria. 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 con- 

 centration 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 chemiotactic 

 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 great function of bacteria is to break up into more 

 simple combinations the complex molecules of the organic 



