42 



CHEMICAL AGENTS AND PROTOPLASM 



[Ch. 1 



be rendered great enough, a repulsion due to osmosis must 

 occur. If the substance is, however, only soluble slightly or 

 miscible, it may be that repulsion -will never occur. Whether 

 or not attraction will occur before the repulsion point is reached 

 will have to be determined experimentally for each reagent. 



Thus the action of an untried substance upon any organism 

 may be any one of three kinds : (1) It may be indifferent at 

 all grades ; (2) it may be indifferent at lower and repellent 

 at higher grades ; (3) it may be indifferent, attractive, and 

 repellent at successive grades. Of two substances belonging 

 to the second or third class, one may act upon an organism at 

 a certain concentration with indifference, the other at the same 

 concentration with repulsion. Likewise the same solution of a 

 substance may attract one kind of protoplasm and repel another, 

 under otherwise similar conditions. 



We have seen above that the same reagent acts upon the 

 same kind of protoplasm similarly only when the other con- 

 ditions of the experiment are also the same. Among the 

 varying conditions which have been especially investigated is 

 that of the chemical constitution of the medium. The experi- 

 ment has generally been made as follows : A particular species, 

 let us say Bacterium termo, is to be subjected to the action of 

 a particular reagent, e.g. meat extract. The bacteria are reared 

 in cultures containing a varying quantity of the meat extract, 

 and the concentration of the capillary fluid producing the 

 threshold stimulation is measured in each case. We may 

 compare not only the threshold stimulations but also the 

 concentrations necessary to produce the response indicated 

 by aj, ftj' ®tc. The following table, from Pfeffeb ('88, 

 p. 634), gives some of such determinations: — 



