IRRITABILITY ; INFLUENCE OF LIGHT 207 



reversal in the direction of movement. The conditions deter- 

 mining such reversals have been described above. 



The photo synthetic effect of light has not been proved. 

 Reasons have been given for , the conclusion that in all 

 probability some form of synthesis does take place through 

 the agency of light. 



The Effect of Chemical Substances in Changing the 

 Direction of Movement. 



Aliyoshi's Experiments. — The table on next page gives the 

 results of this investigator with Pfeffer's Capillary Tube 

 Method. 



The absence of controls in these experiments somewhat 

 impairs the value of these results, but there is no doubt of the 

 sensitiveness of the bacteria to peptone and flesh extract 

 with which the most marked results were obtained. 



Bengt Lidforss found that even very minute quantities 

 of ethyl alcohol attractecj certain bacteria. He also found that 

 a colourless sulphur spirillum was attracted by a dilute 

 solution of sulphuretted hydrogen, but that this compound 

 was poisonous in concentrated solutions. It is inferred that 

 the organism was presumably repelled by more concentrated 

 solutions. A substance may, however, be poisonous to bac- 

 teria and yet exercise attraction for bacteria, as for example 

 corrosive sublimate. 



Sodium thiosulphatc was also found to attract bacteria 

 in dilute solution. 



Molisdi's Experiments. — The results obtained by Molisch 

 gain an added value from his rigorous use of conti'ols. 

 The methods of experimentation were the same as those 

 used by Miyoshi. He estabhshed the important point that 

 ditierent purple bacteria may react differently to the same 

 reagent. Thus Rhodospirillum giganteiim is attracted by 

 weak hydrochloric acid, whilst Rhodospirillum photometricum 

 and Chromatium are repelled. Again, Rhodospirillum gigan- 

 teiim is sensitive to carbon dioxide, hydrochloric acid, 

 dextrin, sucrose, and peptone, whilst Chromatium is in- 



