446 M. E COLLETT 



clearer in graph 1. It will be observed that for Paramoecium 

 at 20° salicylic and HCl are at all concentrations the most toxic 

 of the acids studied and acetic and butyric the least so, with the 

 other acids scattered in between. But although in a general 

 way the toxic order is similar at the different concentrations, 

 there are several important exceptions. For example, the toxic 

 order at 0.0002 N is formic > tartaric > oxalic = malonic, 

 while at 0.0001 N the order is tartaric > malonic > formic > 

 oxalic. This change in order is indicated by intersection of the 

 curves. There is a similar though less important change at the 

 next concentration, where the curve for HCl crosses that of 

 salicylic and the tartaric curve crosses those of malonic and 

 formic. With Euplotes change in concentration produces less 

 variation in order, but instead a considerable variation in the 

 degree of difference existing between the acids. Thus at 0.0005 N 

 HCl formic and benzoic acids kill in 1|, 2|, and 3 minutes, re- 

 spectively, while at 0.0001 N the relative times are 9, 40, and 35 

 minutes. Obviously, if conclusions were based solely upon the 

 results for a single concentration, 0.0005 N for instance, they 

 would not be valid, since the results at other concentrations are 

 in so many respects different. 



This conclusion agrees with the results obtained by Crozier 

 ('16) in studying the rate of penetration of acids into the mantle 

 cells of Stichopus, which contain a natural indicator. He found 

 that the relative order obtaining at a concentration of 0.01 N 

 was in many respects unlike the order at 0.001 N, and that the 

 order of penetration found by Harvey ('14) and by Haas ('16) 

 at 0.01 N with different material closely resembled his results at 

 0.01 N, but not his results at other concentrations. If other 



KEY TO GRAPH SYMBOLS 



1. Hydrochloric 8. Citric 1 



2. Formic — 1-| 1-|— 9. Malonic 



3. Acetic —< < — 10. Tartaric oo 



4. Propionic —«-l <-[— 11. Benzoic 



5. Butyric ^ ' — ^ — 12. Phthalic : . 



6. Lactic 13. Salicylic 



7. Oxalic ^o— — 



