92 F. G. HALL. 



inches long, was closed at one end and fitted with a cap at the 

 other end. In this cap was fitted a glass window four inches 

 in diameter through which the fish could be observed. Pipes 

 running from the fourth floor of the Biology Building were 

 connected with this tank, one carrying water to the tank, one 

 carrying water away from the tank. The height of the overflow 

 pipe was approximately sixty feet above the tank. Thus a 

 pressure was obtained in the tank equal to that at a depth of 

 sixty feet in the lake, or at about two atmospheres greater than 

 at the surface. The fish could be very quickly removed and 

 samples taken for analysis. 



Six controls that were kept in water containing the same 

 amounts of dissolved gases and at the same temperature as the 

 fishes subjected to pressure showed the following averages for 

 gases in their swimbladders : 



Oxygen = 12.1 per cent., Carbon dioxide = 0.65 per cent., 



Nitrogen = 87. 5 per cent. 



The averages for seven perch under sixty feet of water pressure 

 at the end of ten hours duration were : 



Oxygen = 18.5 per cent., Carbon dioxide = 0.50 per cent., 



Nitrogen = Si.opercent. 



This shows that with an increased pressure the percentage of 

 oxygen is increased. 



THE EFFECTS OF INCREASED CARBON DIOXIDE. 



Carbon dioxide is found in very small amounts in the atmos- 

 phere (0.08 per cent.). It is found more abundantly in water 

 on account of its ready solubility. The amount in the water, 

 however, is governed by the partial pressure of the gas in the 

 atmosphere. The free carbon dioxide dissolved in natural 

 waters is affected by photosynthesis and by organic decomposi- 

 tion. 



Carbon dioxide is also found in natural waters in chemical 

 union. It is found in two states (Birge and Juday, 1911): that 

 united with a base in the form of normal- or mono-carbonate such 

 as CaCO 3 or MgCO 3 and is known as "fixed" or "combined" 

 carbon dioxide; and that which converts the monocarbonate 



