110 



Mr. J. Gray. 



The procedure adopted was to determine the critical concentration of 

 hydrogen ions in the external medium, which was just sufficient to cause 

 complete stoppage of ciliary movement in 1 minute. A very large number 

 of experiments were performed, whose results are summarised in the 

 following Table : — 



Acid. 



Critical P H 

 concentration. 



Hydrochloric 

 Sulphuric 



Nitric 



Citric 



3-4 

 3-1 

 3 -4 

 3-4 



Acid. 



Critical P H 

 concentration. 



Oxalic 

 Formic 

 Acetic 

 Butyric 



3 1 

 4-0 

 4-8 



It will be observed that the mineral acids are of practically uniform 

 efficiency, and the same hydrogen ion concentration of each is required to 

 produce the same physiological effect. The fatty acids, on the other hand, 

 form a series which is more efficient than the mineral acids, and the higher 

 member of the series is distinctly more efficient than a lower member. 



It has already been shown (Gray, 12) that, when ciliary activity has 

 ceased in the presence of an acid, recovery takes place when the surrounding 

 medium is made alkaline, so that it is possible to compare the effects of the 

 weak and the strong alkalies as restoratives. 



When movement has been stopped by means of a fatty acid, the cells 

 rapidly recover in a solution which is not more alkaline than normal 

 sea- water (P n 7"8) : by means of gill fragments stained with neutral red, 

 this recovery can be seen to be due to the rapid rate at which the acid is 

 removed from the cell. If, however, the cilia have been stopped by a 

 mineral acid, recovery in normal sea- water is relatively slow (Gray, 12), so 

 that such fragments form satisfactory material for testing the restorative 

 powers of the various alkalies. In the following experiments, fragments of 

 the same gill were exposed to a definite strength of mineral acid (HC1, 

 Pu 3 - 3) for a definite period (5 minutes). Individual fragments were- 

 then transferred to normal sea-water, and to sea-water whose Ph had 

 been raised to the same abnormally high value by NaOH and by NH^OH 

 respectively. 





Sea-water. 



Sea-water. 



Sea-water. 







+ NaOH. 



+ NH 4 OH. 





Ph 7 -8. 



Ph 8 -4. 



Ph 8 4. 





12' 



7' 



1' 





25' 



19' 



3' 



