784 



From what was said above p. 783 line 15 it may be deduced that 

 it (practically) makes no difference, whether the reaction to light, 

 or to gravity or to chemotactics is employed to judge the motility 

 of ChlamydoDionas. A reaction was sought which could easily be 

 followed macroscopically. The gravitational one was found to be 

 the most suitable, for it was easy to ascertain, whether the alga 

 definitely moved towards the bottom, or whether the motility was so 

 small, that there was no question of a downward movement. In the 

 former case a definite accumulation was soon formed at the bottom of 

 the vertical tube ; in the latter case no clear accumulation was observed. 

 The result of the experiment was always noted after ten minutes. 



According as the algae reacted to gravity, or not, it was possible 

 to ascertain, whether the motility in a given solution was fairly 

 large or very small. In all solutions in which the alga reacted to 

 gravity the motility was not uniformly great; nor was this the case 

 in those solutions, in which no reaction could be observed. We 

 coidd, however, determine the limiting concentrations at which the 

 reaction to gravity still occurred and at which it could no longer 

 be observed. By making a series of salt solutions of increasing con- 

 centrations in distilled water, it was possible to determine the con- 

 centration of the salt at which a reaction was still just observable 

 and that, at which a reaction no longer occurred. The concentrations 

 between these two limits may be called transitional concentrations. 

 The mean of the two limits we may regard as the concentration, 

 at which, at least theoretically, the transition took [)lace from a 

 condition of motility in which the reaction to gravity occurred, to 

 a condition of motility in which the reaction no longer took place. 

 This concentration we call \\\e critical transitional concentration ov {\\q 

 critical concentration. 



For solutions in which the salt concentration was constant, whilst 

 the H-ion concentration increased regularly, we similarly speak of 

 the limiting and of the critical concentrations. 



The values of the limiting concentrations become much more 

 certaiji by making each experiment six times. As the two limiting 

 concentrations we regarded that one, at which all six tubes showed 

 a definite accumulatioji and that one, at which no accumulation 

 occurred in any of the tubes. Concentrations at which a positive 

 reaction was found in some only of the six tubes, were regarded 

 as transitional and were left out of account. 



In order to obtain a clear picture of the influence of a salt we 

 must pay attention to the H-ion concentration of the solution. The 

 solutions which were employed in investigating the effect of a given 



