ON THE ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 451 



Sea-water treated with HoO.> also showed a lowering of the alka- 

 linity, hut in a much less degree when, as usual, minimal quantities 

 were used. 



Control experiments on double-distilled water, which had been 

 treated with these substances, were tried, but great difficidty was 

 found in obtaining an end point with the indicator. As far as could 

 be judged, distilled water treated with sol. B (quantities as with sea- 

 water) showed a negative " alkalinity," equivalent to about 8 mgrm. 

 OH 7oo> ^^^^ ^^^ the case of animal charcoal a positive alkalinity 

 equivalent to 6 mgrm. OH °/^^, but the colour change was so slow that 

 these results are only the roughest estimates. The possibility that the 

 above results are due to some effect on the indicator, which entirely 

 cloaks the true alkalinity, must always be taken into consideration. 



Before any attempts at analysis had been made, the probability that 

 considerable differences might be found in the alkalinity of the various 

 media had presented itself. Improvement in the growth of diatom 

 cultures was found to result from the purely empirical addition of 

 NaHCOg, this result being most marked in normal Miquel sea-water, 

 outside water + sol. B only, and Berkefeld water. No growth could be 

 obtained in either tank-water or Miquel sea- water to which had been 

 added 1 cc. HCl (pure, concentrated) per litre, but on again raising the 

 alkalinity of the latter by the addition of NaHCOg or KOH good 

 normal growths resulted. lUchter (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 Tholassiosira decipicns were 

 made in the following media : — 



A. Tank-water. Control. 



Poor groAvtli, hardly uonual. Later, good growth of minute 

 forms, etc. 



B. Tank-water treated with cold commercial animal charcoal and filtered. 



Very good growth indeed. 



C. Tank-water treated with cold pure an. char, and filtered. 



Very poor growth, compara1)le to A without minute forms. 



D. Tank- water treated with pure an. char, as in C, but the an. char, wan 



added red hot. 

 Fair growth, much superior to C, Ijut not up to B. 



The sample of pure an. char, used here had been previously found to 

 give very poor results, and it was also quite contrary to our experience 

 that any improvement in growth should be obtained by adding it hot. 



