3H L. H. HYMAN. 



or greater, depression was produced in normal water, but to a 

 less extent than previously. In the same worms, in carbonate- 

 free water acidified with acetic acid, the depression induced was 

 noticeably less than in the controls. It therefore follows that 

 the depressing action of acetic acid is also in large part due to 

 the carbon dioxide which is sets free. It appears that the 

 penetrating powers of carbon dioxide do not entirely explain its 

 difference from other acids. 



4. Action of Carbon Dioxide Added to Carbonate-free Water. 

 These experiments were designed as a sort of crucial test of the 

 proposition that carbon dioxide is the cause of the depression 

 induced by acidification of natural waters. The proposition was 

 upheld in the most striking manner. The addition of carbon 

 dioxide gas to the same pH causes the same degree of depression, 

 whether added to normal, or to carbonate-free water. It thus 

 appears to be reasonably certain that the depression of the rate 

 of oxygen consumption in acidified water is caused chiefly by 

 carbon dioxide. 



The question then arises: Is the amount of carbon dioxide 

 liberated in carbonate-containing water by the addition thereto 

 of acids, different with different acids at the same hydrogen ion 

 concentration? It became necessary to determine the actual 

 amount of free carbon dioxide present when the well water was 

 acidified with various acids. 



5. Determinations of the Amount of Carbon Dioxide Liberated on 

 Acidification of the Unaltered Water. The normal well water, 

 pH 8.0, was acidified to the desired pH with various acids. A 

 sample of 100 cc. was then immediately drawn and the free 

 carbon dioxide in it determined by titration with N/$o barium 

 hydroxide, properly protected from the carbon dioxide of the air, 

 using phenolphthalein as indicator. 



Hydrochloric, sulphuric, acetic, and butyric acids were inves- 

 tigated at ranges of pH 7.5 to 4.0. It was found that the amounts 

 of carbon dioxide liberated from the carbonates of the water by 

 different acids are similar, but not identical, at the same pH. 

 Such differences probably account for some of the different 

 percentages of depression obtained with different acids at the 

 same pH. But butyric acid was also found to liberate nearly as 



