REACTIONS OF FISHES TO ATMOSPHERIC GASES 225 



Weigelt worked with ammonia (NH3) and reports that 10 to 17 

 mg. of ammonia per Hter had no effect on small fish and even 

 30 mg. per liter did not affect large ones, but smaller ones were 

 affected by less than this amount. Fourteen milligrams are not 

 uncommon in sewage, while 43 mg. have been reported. 



V. REACTION OF FISH TO GASES IN SOLUTION 

 1. METHOD OF EXPERIMENTATION 



It is impossible to study the reaction of fishes to gases with- 

 out first establishing a gas gradient. Long boxes 120 cm. by 

 14 cm. by 20.5 cm. with screen partitions 5 cm. from the ends 

 were constructed (figs. 2 and 3). The drain D placed at the 

 center near the top consisted of a tube with screen bottom, 

 opening outside. Water was allowed to flow in at both ends at 

 the same rate (usually 600 cc. per minute) through tees perfor- 

 ated so as to distribute the flow across the tank (fig. 3). The 

 two currents, too slight to interfere with the behavior of the 

 fishes, met at the center of the box. The temperature and flow 

 of the water into the ends must be the same if the gradient is 

 to be perfect.^ 



Water which has been treated in the gas control apparatus, 

 was introduced at the end A (fig. 3) so that a gradient was estab- 

 lished in tank AB, while the control tank BB was alike at both 

 ends. The oxygen content of the water in a typical boiled water 

 gradient is shown in figure 2. The water coming from the tee 



^ Tap water was introduced from |-inch cocks into the tee (T) introducers at 

 the end of the tanks marked {B, fig. 3) and at the same rate into end A from the 

 gas control apparatus. The flow of water was regulated by shoving the rubber 

 tubing connecting the tee introducer with the hose-end, onto the hose-ends as 

 far as possible and ligaturing them securely with copper wire. Each rubber tube 

 was wrapped with bicycle tape for a distance of about 3 cm. on each side of the 

 termination of the metal hose-end. Another ligature was usually applied over the 

 tape. A screw pinchcock was then placed over the wrapped portion of eachtube. 

 With the metal valve wide open each pinchcock was screwed down until the desired 

 flow was secured; by measuring twenty seconds flow in a graduate the rate of flow- 

 was determined. The flow does not have to be adjusted more than once or twice 

 a day and is almost constant for several hours. A metal valve is set for a given 

 flow with much difficulty and the flow soon falls off because of the accumulation 

 of sediment in the valve. 



