400 K. J. ALr,EN AND K. W. NELSOX. 



Before any attempts at analysis had been made, the proba- 

 bility that considerable differences might be found in tlie 

 alkalinity of the various media had presented itself. Im- 

 provement in the growth of diatom cultures Avas found to 

 result from the purely empirical addition of NaHCO.^, this 

 result being most marked in normal Miquel sea-water, outside 

 water + solution B only, and Berkefeld water. No growth 

 could be obtained in either *' tank-water " or Miquel sea- 

 water to which had been added 1 c.c. HCl (pure, concentrated) 

 per litre, but on again raising the alkalinity of the latter 

 by the addition of NallCO., or KOH, good normal growths 

 resulted. Richter (18) and H. Gill (5), also, both state 

 that a weak alkaline reaction is necessary for the growth of 

 diatoms. 



In our most recent experiments, all the media have been 

 analysed for alkalinity, and those given in detail below 

 illustrate the importance of determining this factor. Cultures 

 of Thalassiosira decipiens were made in the following 

 media : 



A. Tank-water. Control. 



Poor growth, hardly normal. Later, good growth of 

 minute forms, etc. 



B. Tank-water, treated with cold commercial animal char- 



coal, and filtered. 

 Very good growth indeed. 

 c. Tank-water treated with cold, pure animal charcoal, and 

 filtered. 

 Very poor growth, comparable to a without minute 

 forms. 

 D. Tank-water treated with pure animal charcoal as in c, 

 • but the animal charcoal was added red-hot. 

 Fair growth, much superior to c, but not up to B. 

 The sample of pure animal charcoal used here had been 

 previoush' found to give very poor results, and it was also 

 quite coutrai'y to our experience that an}' improvement in 

 growth should be obtained by adding it hot. But if we 

 examine the results of analysis of these media for alkalinity 



