26 COMPARATIVE PHYSIOLOGY 



effect the pH. limits with both types of acid would be expected 

 to be the same. A number of experiments were carried out 

 by Gray to determine the critical concentration of hydrogen 

 ions in the external medium which would produce arrest in 

 one minute. The results are given below. 



It will be noticed that the mineral acids are of practically 

 uniform efficiency ; while the fatty acids form a series of which 

 the higher members are more efficient, all being more efficient 

 than the mineral acids. This result strongly suggests that the 

 acid enters the cell, since the effect is related not to the absolute 

 hydrogen ion concentration of the medium but the penetrative 

 power of the acid employed. The value of those observations 

 are, moreover, reinforced by considering the phenomenon of 

 recovery of pieces of gills exposed to acid for the same length of 

 time, when transferred to sea- water made alkaline with weak 

 and strong alkalis. The time (in minutes) for recovery in 

 such an experiment is given in the following protocol : — 



The experiment may be varied by determining the time for 

 recovery on transference to sea-water raised to a known 

 alkalinity by addition of NaOH and NH4OH, respectively, as 



NaOH. 



5 



7 



8 



8-10 



10-12 



12-15 



To sum up in Gray's own words, *' The weak acids which 

 enter the cell are more efficient inhibitors of ciliary movement 

 than the strong acids which do not enter readily, and con- 

 versely the weak alkalis are much more efficient restoratives 

 than the strong alkalis.'* 



