32O L. H. HYMAN. 



I 



9. The depressing action of acidified natural waters on the 

 rate of oxygen consumption of Planaria is not the same with 

 different acids at the same hydrogen ion concentration. This 

 appears to be due largely to the fact that the amount of carbon 

 dioxide immediately liberated from the carbonates of the water 

 differs with different acids at the same pH. 



10. The depressing action of acids in natural waters is greater 

 the greater the acidity up to an acidity of about pH 5.0. This is 

 due to the fact that the more acid added, the greater is the 

 quantity of carbon dioxide liberated. 



11. Lower concentrations are, however, relatively more effec- 

 tive than higher ones. 



12. The maximum amount of depression of oxygen con- 

 sumption that can be induced by acids is about 50 per cent. 

 This occurs at pH 4.0 to 5.0 and further acidification of the water 

 does not increase the percentage of depression. At the acidity 

 at which the maximum depression appears, the carbon dioxide 

 content is about 3 per cent. 



13. All of the carbonates of the fresh water employed are 

 decomposed by acids at a pH of 4.0 to 5.0, producing a carbon 

 dioxide concentration of 3 per cent. This might explain the facts 

 given in 12 w^ere it not that concentrations of carbon dioxide gas 

 much higher than 3 per cent, do not increase the percentage of 

 depression beyond 50 per cent., as long as the oxygen supply is 

 ample. Concentrations of carbon dioxide gas up to 25 per cent, 

 were tested. 



14. From the facts cited in 13 it appears necessary to assume 

 that the depression of the rate of oxygen consumption which can 

 be induced by carbon dioxide does not exceed 50 per cent, as 

 long as the oxygen supply is adequate. 



15. A combination of high carbon dioxide content and low 

 oxygen content practically abolishes the oxygen consumption of 

 Planaria, even though the oxygen content used (2 cc. per liter) 

 would be ample for normal respiration in the absence of carbon 

 dioxide. 



16. No explanation has been discovered for the differences 

 between the action of carbon dioxide and other acids at low 

 concentrations (pH 7.5 and 7.0). 



