§3] 



CHEMOTAXIS 



41 



Compare also the table on p. 37. 



For all reagents which exert an attractive influence there 

 exists the maximum (repelling) and minimum (indifferent) 

 limits referred to. In the case of reagents, which, like alco- 

 hol, repel bacteria at between 1% and 10%, there is doubtless 

 an indifferent limit, but it is not necessary that there should be 

 a degree of concentration at which attraction takes place. In 

 the one case, then, the phenomena of indifference, attraction, 

 repulsion, follow each other with increasing concentration ; in 

 the other case, only indifference and repulsion. The difference 

 in action of the two cases is due, in part at least, to the fact 

 that all solutions, independently of their chemical constitution, 

 become repellent when they become concentrated enough. 

 The repulsion, then, of high grades of chemical solutions is 

 purely an osmotic phenomenon, and, as such, will come under 

 discussion in the third chapter. It follows, also, from what has 

 been said, that, in the case of those reagents which exert no 

 attraction at any concentration, the acme and maximum coin- 

 cide and lie at the saturation point of the solution. 



Finally, we may discuss the third case in which the reagent 

 acts indifferently, as glycerine does upon bacteria between 17 % 

 and 0.86%. It is clear, that if the density of the solution can 



