VITAL PHENOMENA OF BACTERIA. 59 



Some chemical substances seem to exert a peculiar 

 attraction for bacteria, known as positive chemotaxis, 

 while others repel them negative chemotaxis. More- 

 over, all varieties are not affected alike, for the same 

 substances may exert on some bacteria an attraction 

 and on others a repulsion. Oxygen, for example, 

 attracts aerobic and repels anaerobic bacteria, and for 

 each variety there is a definite proportion of oxygen, 

 which most strongly attracts. The chernotaxic prop- 

 erties of substances are tested by pushing the open end 

 of a fine capillary tube, filled with the substance to be 

 tested, into the edge of a drop of culture fluid contain- 

 ing bacteria and examining the hanging drop under 

 the microscope. We are able thus to watch the action of 

 the bacteria and note whether they crowd about the tube 

 opening or are repelled from it. Substances showing 

 positive chemotaxis for nearly all bacteria are peptone, 

 urea, and very weak solutions of bichloride of mercury. 

 While among those showing negative chemotaxis are 

 alcohol and many of the metallic salts. 



The Production of Light. Bacteria which have the 

 property of emitting light are quite widely distributed 

 in nature, particularly in media rich in salt, as in sea- 

 water, salt fish, etc. Many of these, chiefly bacilli and 

 spirilla, have been accurately studied. The emission of 

 light is a property of the living protoplasm of the bac- 

 teria, and is not usually due to the oxidation of any pho- 

 togenic substance given off by them ; at least only in two 

 instances has such substance been claimed to have been 

 isolated. Every agent which is injurious to the exist- 

 ence of the bacteria affects this property. Thus, cold 

 paralyzes them and interrupts their power of emit- 

 ting light. High temperature, acids, chloroform, etc., 



