RELATION TO ENVIRONMENT 143' 



being positively chemotaxic; other chemical substances repel 

 the bacteria and are spoken of as negatively chemotactic. 

 Pfeiffer found that the salts of the alkalies exert a very posi- 

 tive effect ; that the alkaline earths exert a less active in- 

 fluence. Inorganic acids are generally repellent, except 

 phosphoric acid, and the phosphates of the muscle. Or- 

 ganic compounds are frequently attractive. This might 

 readily be expected, because it has been so frequently observed 

 that decomposing organic matter is attractive to bacteria, 

 and this might, in all likelihood, be attributed to organic sub- 

 stances. Many of the organic acids are attractive. Such 

 nitrogen compounds as urea, aspargin, peptone, and meat ex- 

 tract are very attractive. Glycerine is neutral or without 

 effect, while alcohol produces repellent action. 



The different concentrations of solutions produce different 

 effects; potassium chloride attracts in dilute solutions but 

 repels in stronger solutions. It is impossible to predict what 

 effect any chemical substance may have on the bacteria on the 

 basis of its chemical constitution. The action is appar- 

 ently not determined by the elements which enter into the 

 compound, but by the entire compound ; for instance, milk 

 sugar produces a positive chemotaxic effect in one per cent 

 solutions, while grape sugar requires a ten per cent solution. 

 It is conceivable that this phenomenon may be of use to the 

 bacteria, for instance, in helping them to select their food; but 

 it appears that food substances are not always positively 

 chemotaxic, and the bacteria have no power, apparently, of 



