444 M. E. COLLETT 



This is not a satisfactory procedure, as was pointed out by Cro- 

 zier in his work upon permeability. Lack of uniformity in 

 concentration and temperature makes a comparison of different 

 experiments almost impossible, even when the reactions involved 

 are the same, and there are relatively few experiments in which 

 several kinds of material or several physiological processes have 

 been studied all together. The aim of the present experiments 

 is to determine the relative toxicity of a series of acids, at sev- 

 eral concentrations each and at various temperatures, to differ- 

 ent sorts of nearly related material, and from these more com- 

 plete experiments to draw what conclusions are possible as to the 

 nature of the toxic action. I am greatly indebted to Dr. M. H. 

 Jacobs, who suggested the problem, for helpful advice and 

 criticism. 



METHOD 



The organisms observed in the following experiments were 

 Paramoecium caudatum, Stylonichia pustulata, Euplotes pa- 

 tella, and Vorticella nebulifera. They were grow^n in small jars 

 of boiled hay infusion made with pond-water and seeded with 

 decaying leaves, etc. Though many forms appeared in the cul- 

 tures, only these four were constantly present in sufficient num- 

 bers to be useful for a long series of experiments. 



The acids used were hydrochloric, formic, acetic, propionic, 

 butyric, valeric, lactic, oxalic, malonic, tartaric, citric, benzoic 

 salicylic, and phthalic. All were made up at 0.1 N and titrated 

 with litmus against 0.1 N NaOH. These stock solutions, as 

 well as the weaker solutions made from them for use in the ex- 

 periments, were kept in paraffined bottles, for they deteriorate 

 rapidly in unprotected reagent bottles, even those prepared by 

 thorough steaming. 



The organisms were always washed overnight before use in 

 order to eliminate fluctuations due to the changing composition 

 of the culture medium. They were collected from the culture 

 in as concentrated masses as possible and pipetted into about 

 twenty times their volume of fresh pond-water. After twelve 

 hours' washing they were concentrated again by means of a 



