CONDITIONS AFFECTING BACTERIAL MOTILITY. 25 



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 move- 

 ments become more active if the temperature be raised. 

 Most interest, however, attaches to movements which, from 

 the use of an unscientific terminology, are often described 

 as if they were purposive, that is when the bacilli are 

 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. His method was to introduce the up- 

 turned point of a copper rod into a drop of fluid containing 

 the bacteria and suspended from the lower surface of a 

 cover-glass. On warming the outer end of the rod, heat 

 waves, of course, were conducted up to the point and 

 the bacteria swarmed round the latter. Most important 

 observations have been made on the attraction and re- 

 pulsion 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, and, introducing it into 

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

 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 attractive bodies, and in comparing organic 



