28 Chapter I 



movements of Bacteria under the influence of increasing stimulations, 

 has established the fact that, conformably to this law, when the 

 stimulus increases in geometrical ratio, the irritability increases in 

 arithmetical ratio, that is to say, the reaction is proportional to 

 the logarithm of the stimulation. In order that a motile bacterium 

 {Bacterium termo), grown in a peptonised solution, may perceive a 

 difference of medium, it is necessary to place it in a peptone solution 

 of five times the original concentration ; weaker solutions, in which the 

 concentration is but three or four times greater than the original 

 fluid, do not attract the bacteria at all ; consequently these differ- 

 ences are below their chemiotactic sensibility. 



The different reactions that are exhibited in the immunity of 

 unicellular organisms, reactions which are dependent on the irrita- 

 bility of their protoplasm, therefore, come undeniably under the 

 category of purely cellular phenomena. 



