ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 395 



some accidental impurity in the salts or distilled watei* from 

 which the successful media were prepared. 



Alkalinity. — Torncie (43) and Dittmar (33) were the first 

 to investigate the fact that sea-water showed on analysis an 

 apparent excess of base over acid, which excess they termed 

 ''the aliialinity of sea-water." Dittn^ar defines the alkalinity 

 of sea-water as "a measiii-e of its potential carbonate of 

 lime," but this definition and his supposition that this excess 

 of base combines directly with dissolved COo to form car- 

 bonates and, further, but only in veiy small proportion, bicar- 

 bonates, is liable to give a quite erroneous idea of the state of 

 equilibrium actually occurring in the ocean. For, as Fox (34) 

 has shown, " sea-water reacts in situ very nearly neutral, and 

 actually just slightly more acid than distilled water." This 

 is due to the fact thnt sea-water always contains a consider- 

 able quantity of dissolved C0.2- 



If a salt solution with neutral reaction, that is, containino- 

 H* and OH^ ions in concentrations equal to one another and 

 the same as for pure water, be exposed to an atmosphere con- 

 taining COo, a definite amount, depending on pressure, tem- 

 perature, and salinity, would go into solution. This CO.^ 

 would combine with water and form the very weak acid 

 H^CO.;, which would ionise with the formation of free H* ions, 

 thus : 



H.CO, --^ H- + HCO', 



(HCO', ~Z H- + CO'y 

 The second stage of dissociation is so small as to be 

 negligible. The concentration of H* being now increased 

 and OH^ decreased, the solution would have an acid reaction. 

 The actual amount of COo thus dissolved would always be 

 small ; for instance, a salt solution of strength CI = 20*00 

 (average Atlantic water CI = 19-4) will at 10° C. dissolve 

 about "3 c.c. CO3 per litre from an atmosphere containing 

 3 %oo C0._, (about normal). But the ocean is found to contain 

 very much greater quantities than this, 60 c.c. or 200 times 

 this amount being a not unusual figure for the total CO. 

 The difference between this amount and the '3 c.c. or so dis- 



