REVERSION IN ORIENTATION TO LIGHT 515 



ascertained. The. solution gave a definite acid reaction with 

 neutral red. The following day all of the colonies were dead and 

 the solution was still distinctly acid. The fact that the time 

 required for positive individuals to become negative is greatly 

 extended when the proper amount of acid is added, seems to 

 indicate that under certain conditions the colonies may be 

 permanently positive. I was, however, unable to obtain such 

 conditions. 



While it has thus been clearly demonstrated that concentration 

 is a factor in reversion, the following experimental results demon- 

 strate equally clearly that the time rate of change in the concen- 

 tration of the effective elements in the solution is also a factor. 



EFFECT OF TIME-RATE OF CHANGE IN CHEMICAL CONCENTRATION 



On December 5 numerous colonies in 1 cc. of solution were 

 taken from an old culture from which considerable water had 

 evaporated and exposed in diffuse light. The colonies were 

 strongly negative. Hydrochloric acid was now added in suffi- 

 cient quantity to make the solution n 1000 HCl on the basis of 

 pure water. The HCl was now increased step by step through 

 the following concentrations: n 500, n/333, n 25(1, n/200, n/166, 

 n/142, n/125, n 111, n/100. During all this addition of acid 

 there was at no time any indication of a reversal to positive 

 reactions. The colonies were continuously definitely negative. 

 But when colonies were taken from the same jar and put directly 

 into the n '200 solution or into any of the solutions above this, 

 they became strongly positive, remained so for several minutes, 

 and then became negative again. 



These results show clearly that the effect on reversion in 

 orientation in 8pondylomorum of slowly adding a certain amount 

 of acid to a gi^'en amount of culture fluid is very different from the 

 effect produced by rapidly adding the same relative amount of 

 acid. We have previously demonstrated that the effect of acid 

 on re\'ersion is in all probability associated with the accompanying 

 reduction in hydroxyl ions. If this is true, it may be concluded 

 from the results now under consideration that it is dependent upon 

 the time-rate of change in the concentration of the hydroxyl 

 ions. 



