OXYGEN CONSUMPTION IN ACIDS. 319 



concentrations, it follows that the free hydrogen ions cannot be 

 the chief factor in toxicity. The penetrating powers of the acid 

 are probably of great importance and the nature of the anion or 

 molecule may also be involved. Butyric and acetic acids which 

 probably penetrate organisms the most readily of all the acids 

 employed are also the most toxic. In a study of the toxicity of 

 various acids for ciliate Protozoa, Collett ('19) reached the 

 conclusion that hydrogen ion concentration is not the most 



important factor. 



SUMMARY. 



1. The effect of acidification of the medium on the rate of 

 oxygen consumption of aquatic organisms was studied. 



2. The acids used were: hydrochloric, nitric, sulphuric, car- 

 bonic, butyric, acetic, citric, and tartaric. They w r ere added to 

 w^ater to produce acidities ranging from pH 7.5 to 5.0, at intervals 

 of 0.5 pH. 



3. Planaria dorotocephala was the chief animal used as material. 

 Some tests with hydrochloric acid were also made using starfish 

 and nudibranchs. 



4. The acidification of natural waters, either salt or fresh, 

 (pH 8.0), by any of the acids used except butyric causes a 

 decrease in the rate of oxygen consumption at all acidities 

 greater than pH 7.0. The majority of acids also cause depression 

 between 7.8 and 7.0 but in the case of acetic acid and more 

 doubtfully carbonic, there was some tendency towards a slight 

 acceleration of the rate of oxygen consumption at these lower 

 concentrations. 



5. The decrease in the rate of oxygen consumption due to 

 acids is completely and promptly reversible, as long as the 

 animals are not actually injured. 



6. The acidification of fresh water from which all carbonates 

 have been previously removed has no or only a slight effect upon 

 the rate of oxygen consumption of Planaria, except when the 

 acidity is produced by carbon dioxide. 



7. The depressing action of carbon dioxide is the same w r hether 

 the gas is added to ordinary or to carbonate-free water. 



8. From 6 and 7 it follows that the depressing action of acids in 

 natural waters is due chiefly or wholly to the carbon dioxide 

 which they liberate from the carbonates of such waters. 



