SEA- FISHERIES LABORATORY. 263 



4. The " Reactivity " effect is mainly produced by 

 dissolved magnesium bi-carbonate, and not calcium bi-carbonate 

 as usually stated, 



5. In all cases the normal fresh sea- water gave a pink 

 colour with phenol-phthalein indicating a potential of hydrogen- 

 ion concentration lying below P Hf 10~ 8 , the average being 

 about P„, 10 -* 2 . 



6. The seasonal variations in P H have been followed 

 out by the colorimetric method of Sorensen and by the 

 Hydrogen Electrode method, and a small but distinct increase 

 in alkalinity in Spring has been detected. 



7. This vernal increase in alkalinity is not due to 

 increasing temperature disturbing the equilibrium between 

 the carbon-dioxide of sea-water and atmosphere, for the rise in 

 alkalinity clearly precedes in time the rise in temperature. It 

 is caused by photo-synthesis as is shown by its coincidence in 

 its occurrence with the rapid lengthening of the day in March 

 and the increasing sun's altitude, as also by the great changes 

 in alkalinity which may be produced by exposure of sea-water 

 containing algae to sunlight. 



8. Algae continue abstracting carbon-dioxide and so 

 increasing alkalinity until all the bi-carbonates have become 

 changed into normal carbonates, and then definitely cease to 

 functionate and rapidly die at this latter point. The potential 

 of hydrogen-ion concentration falls to P H , 10~ ,J before synthesis 

 ceases. At this point the sea-water gives an intense pink to 

 phenol-phthalein, and titration gives a figure almost exactly 

 half that of the total reactivity efEect. 



We desire to express our indebtedness to the Percy Sladen 

 Memorial Trust for the necessary funds for this Research which 

 were allotted to one of us by the Trustees. 



