SEA-FISHERIES LABORATORY. 257 



It thus became of interest to determine the limits to 

 which, under the most favourable circumstances of a smaller 

 volume of sea-water exposed to light in presence of green algae, 

 such increase of alkalinity and depression of hydrogen-ion 

 concentration could be carried. 



The fact that sea-water is usually alkaline to phenol- 

 phthalein is sufficient to show that, save for an infinitesimal 

 amount of dissolved carbon-dioxide requisite to keep up 

 potentially the partial pressure of carbonic acid anions in the 

 water, all the carbonic acid is present as a bi-carbonate along 

 with a small fraction as normal carbonate. 



When photo-synthesis commences this small potential 

 amount of carbon-dioxide is synthesised into organic carbon 

 compounds, this change upsets the equilibrium and so the 

 bi-carbonate (of magnesium) breaks up yielding a supply which 

 restores the tension in solution of carbonic acid. The question 

 is, how far can this process go before the rising alkalinity 

 destroys the algae, and the answer is beautifully given by 

 the following experiments. 



It has already been shown by Nathansohn 14 that aquatic 

 plants can assimilate perfectly in water containing bi-carbonates, 

 but not in water containing only normal carbonates ; but as 

 far as we are aware, the exact point of stoppage in a natural 

 mixture of carbonates and bi-carbonates such as is present in 

 sea-water, or the value of the hydrogen-ion concentration at 

 the end of the process, have not hitherto been determined. 



The result is interesting — the photo-synthetic action 

 stops at the precise point where all the available bi-carbonate 

 has been converted into carbonate, and the hydrogen-ion 

 concentration has then reached the surprisingly low value 

 indicated by the potential P H> 10 " 9, \ 



Experiment 1. August 29th, 1912. A sample of sea- water 

 was taken at the "Life-boat Slip" and this was carefully 

 neutralised by adding the calculated amount of centi-normal 



