292 L. H. HYMAN. 



than carbon dioxide content for aquatic organisms. The facts 

 that Jewell ('22) found an extensive and varied fauna in an acid 

 stream (pH 5.8 to 7.1) and Jewell and Brown ('24), several 

 fishes living in an acid lake (pH 4.4) indicate that hydrogen ion 

 concentration is not of paramount importance in the distribution 

 of aquatic animals. Alice ('23) noted a lack of relation of 

 hydrogen ion concentration to the distribution of marine animals. 



From the investigations cited the general conclusion is certainly 

 justifiable that the acidification of the medium to the proper 

 extent retards or depresses biological processes and activities. 

 My own experiments are in agreement with this general result. 

 But without further analysis the conclusion should not be drawn 

 that the observed effect is assignable to the free hydrogen ions of 

 the acidified medium. Natural waters usually contain salts 

 which undergo chemical changes when the water is acidified. 

 In particular, carbon dioxide is evolved. Unless it is definitely 

 proved that the evolution of carbon dioxide or other secondary 

 chemical changes attendant on acidification of natural waters is 

 not concerned in the result, the observed effect should not be 

 assigned to the hydrogen ion concentration per se. Even ordi- 

 nary distilled water contains some carbonates and other salts 

 which may affect the result. Some of the researches cited in 

 the foregoing review cannot be criticized on this score, or only 

 to a slight extent, as distilled water was used, but in other cases, 

 where natural waters were employed, the carbon dioxide evolved 

 was probably of more consequence in the result observed than 

 the hydrogen ion concentration. 



In an extensive study of the depressing effect of acidification 

 of natural waters on the oxygen consumption of aquatic animals, 

 presented in this paper, it is shown that in all probability, carbon 

 dioxide is chiefly responsible for the effect. 



METHODS. 



The general course of procedure in the experiments to be 

 reported was as follows. The rate of oxygen consumption of 

 the animals in normal water was determined for a definite 

 period of time, generally one hour. The normal water was then 

 acidified to the desired extent and the oxygen consumption in 



