OXYGEN CONSUMPTION IN ACIDS. 317 



6. Conclusions Concerning the Experiments with Carbonate-free 

 Water. (a) The depressing effect of acids on the oxygen con- 

 sumption of Planaria is almost wholly abolished when carbonate- 

 free water is used, except when the acidity is produced by carbon 

 dioxide gas. 



(&) An acidity due to carbon dioxide gas is equally depressing 

 in ordinary and in carbonate-free water. 



(c) From the statements in (a) and (6) it is concluded that the 

 depressing action of acids on oxygen consumption is almost 

 wholly due to the carbon dioxide liberated on acidification of 

 natural waters. 



(d) Different acids at the same hydrogen ion concentration do 

 not immediately liberate the same amounts of carbon dioxide from 

 the natural carbonate-containing water. This accounts in large 

 part for the fact that the depressing action of different acids is 

 not the same in degree at the same pH. 



(e) All of the carbonate of the water used is decomposed at a 

 pH of 4.0 to 5.0, giving a carbon dioxide content of about 3 

 per cent. 



(/) The lack of action of butyric acid is not explained, for 

 this acid also liberates carbon dioxide from the carbonates of 

 the water. Possibly butyric acid in some way prevents the 

 penetration of carbon dioxide into the animals. 



(g) The amount of carbon dioxide gas required to produce a 

 given pH is considerably greater at acidities of more than pH 6.5 

 than is the amount of carbon dioxide liberated in the water by 

 other acids at the same pH. One would therefore expect that 

 COo would be much more effective as a depressing agent at 

 acidities from 6.5 to 5.0 than any other acids. This was not 

 noticeably the case. The difficulty could be explained by 

 assuming that the depressing action of COo reaches a maximum 

 at a concentration of 3 per cent, and is not altered by further 

 increase in the concentration of the gas. 



(h) Since the depressing effect of acids is due largely to carbon 

 dioxide and since this gas is an end product of metabolism, it 

 becomes readily understood that organisms or parts must be 

 affected by acids in relation to their respiratory metabolism. 

 The higher the rate of the latter, the greater would be the depres- 

 sion induced by acids. 



