514 S. O. MAST 



sion, it is certain that reversion is not specifically associated with 

 their concentration, for organisms were repeatedly observed at 

 different times to be negative as well as positive in practically 

 all concentrations in which they oriented at all. Thus while the 

 organisms tend to be positive in relatively weak and negative 

 in relatively strong alkaline solutions, in four different experiments 

 they were actually observed to be negative in solutions which 

 were clearly slightly acid. Moreover, as may be observed by 

 referring to the detailed description given above, of the effect of 

 acids on the sense of orientation, the colonies change from positive 

 to negative orientation without any appreciable change in the 

 chemical constitution of the environment. This clearly shows 

 that the sense of orientation is not specifically dependent upon the 

 hydroxyl ion-concentration. Reversion is, however, not wholly 

 independent of concentration. 



EFFECT OF CHEMICAL CONCENTRATION 



If acid, as previously stated, is added to a solution containing 

 negative colonies they become positive, but after a certain time 

 they become negative again without any further change in the 

 solution. The time required for the reversion from positive to 

 negative orientation depends upon the amount of acid added. 

 For example, in one experiment, with sufficient hydrochloric 

 acid added to make the culture fluid n/ 10,000 HCl, on the basis 

 of pure water, it required approximately 3 minutes; with sufficient 

 added to make it n 500 HCl, it required 3 minutes; with sufficient 

 to make it n/333 HCl, it required 5 minutes; with sufficient to 

 make it n/250 HCl, it required 6 minutes; with sufficient to make 

 it n/200 HCl, it required a little over 8 minutes, and with sufficient 

 to make it n/167 HCl, it required 84 minutes. In another ex- 

 periment with sufficient hydrochloric acid added to make the 

 culture solution n/1000 HCl, the colonies did not become posi- 

 tive at all; with sufl^cient added to make the solution n/200 HCl, 

 they become definitely positive, and it required 6 minutes to be- 

 come negative; with enough acid added to make n/143 HCl, they 

 remained strongly positive for nearly one hour and then became 

 neutral; whether or not they became negative later was not 



