246 



MORRIS M. WELLS 



had been made at Chicago, by determmmg the conductivity of 

 the water at various points in the tank, tests were no longer 

 made. Thus the actual concentrations existing throughout the 

 tank have not been determined in each experiment but the study 

 that was inade indicated very clearly that under the given con- 

 ditions this concentration is almost constant for a given salt. 

 Thus there always exists a gradient of the dissolved salt, between 

 the two ends of the tank. The presence of this gradient is 

 shown by the reactions of the fishes as well as by the conductivi- 

 ties and titrations. That the gradient is not perfect is to be 

 expected; its peculiarities were brought out in the study which 





l-_^_- 



_i— ^ 





f-- 



r 



Fig. 1 Longitudinal section through the gradient tank. The figures indicate 

 the concentrations of the salt at the depths indicated by the arrows. These 

 concentrations were ascertained by determining the conductivity of samples 

 taken from the different parts of the tank; in determining the gradient 7 samples 

 along any given level were taken; only five are shown in the figure. 



was made by means of the conductivity method. Figure 1 

 shows the gradient as it existed after the flows at the ends had 

 been running for some time. 



It will be noted from figure 1 that at any given level there is 

 a gradient of salt from end to end of the tank. The concentra- 

 tion at the bottom of the tank was much higher than that near 

 the surface of the water, and thus the fishes at times reacted to 

 the vertical gradient, which was much sharper than the hori- 

 zontal one. This reaction to the vertical gradient did not inter- 

 fere greatly with the experiments, however, because the fishes 

 tend to swim back and forth in the tank at whatever level they 

 may be. Furthermore, most of the fishes worked with, remained 

 near the bottom for a large proportion of the time. A further, 



