ON THE ARTIFICIAL CULTUKE OF MARINE PLANKTON ORGANISMS. 447 



The only success we attained with artificial sea-water as a basis for 

 culture media was with four isolated cultures in one of our earlier 

 experiments. Two of these were cultures of Coscinodiscus cxcentricus 

 in artificial sea- water + Miquel's solutions A and B. The two cultures 

 were identical except that one was in an ordinary Bohemian glass 

 flask and the other in a " resistance glass " flask. No difference be- 

 tween these two could be seen. The growth obtained in both was in 

 every way equal to normal Miquel sea-water, and is still fair, although 

 over two years old. The other two successful cultures were growths 

 of the same diatom in the same media, plus a small quantity of weed 

 infusion, made by boiling up a small piece of Ulva in artificial sea- 

 water. These gave just as good results, but the addition of unknown 

 factors from the weed detracts from their general interest. In spite 

 of frequent attempts, over fifty in number, we have not been able to 

 repeat this experiment, which may possibly be due to some accidental 

 impurity in the salts or distilled water from which the successful 

 media were prepared. 



Alkaliiiity. Tornoe (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 alkalinity 

 of sea-water." Dittmar defines the alkalinity of sea-water as " a 

 measure of its potential carbonate of lime," but this definition, and 

 his supposition that this excess of base combines directly with dis- 

 solved CO2 to form carbonates and, further, but only in very small 

 proportion, bicarbonates, 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, w situ very nearly neutral, and 

 actually just slightly more (Kyid than distilled water." This is due 

 to the fact that sea-water always contains a considerable quantity 

 of dissolved COo. 



If a salt solution with neutral reaction, that is containing H* and 

 OH' ions in concentrations equal to one another and the same as 

 for pure water, be exposed to an atmosphere containing COo, a definite 

 amount, depending on pressure, temperature, and salinity, would go 

 into solution. This CO^ would combine with water and form the very 

 weak acid H2CO0, which would ionize with the formation of the 

 free H* ions thus : — 



HoCOg'? H'-hHCOV 

 (HCO'3 ? H' + CO".). 



The second stage of dissociation is so small as to be negligible. The 

 concentration of H* being now increased, and OH' decreased, the 



