ON THE ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 443 



protective, is better postponed until a later section, after the action of 

 animal charcoal and other substances has lieen considered (see p. 455). 



Animal Charcoal and Peroridc of Hydrogen. The most successful 

 culture medium for plankton diatoms, next to Miquel sea-water, is 

 that prepared from animal charcoal (cf. p. 435). Animal-charcoal 

 water gives at first almost as good cultures of plankton diatoms as 

 Miquel sea-water, but the tendency to paleness and exhaustion 

 appears much sooner. The best cultures were obtained in " Berkefeld 

 water," that is, tank-water from the Plymouth Aquarium treated with 

 powdered commercial animal charcoal and filtered through a Berkefeld 

 filter. Tank-water as a basis for animal-charcoal water is very much 

 better than outside water, probably on account of the higher con- 

 centration of nitrates, etc. 



There is a very striking resemblance between the effect of animal 

 charcoal and of Miquel's sol. B upon sea-water used for diatom 

 cultures, and the growths obtained by using tank-water + sol. B and 

 tank animal-charcoal water are very similar in character. If Miquel's 

 sol. A is added to animal-charcoal water, there is a great improvement, 

 both in the colour and quantity of diatom growth, and in the case of 

 Thalassiosira decipiens the chains are long and well formed. With 

 animal - charcoal water + sol. B, on the other hand, practically no 

 growth was obtained. 



It is possible that a certain amount of phosphate, and perhaps of 

 calcium, from the animal charcoal, goes into solution and serves as a 

 " nutritive " material for the diatoms. But we are inclined to think 

 that its chief action is " protective," and due to its power of occluding 

 gases, such gases being in a state of higher chemical activity than 

 under normal conditions.* 



As was explained in a previous section (p. 434), the possibility that 

 the action of animal charcoal might have some sort of effect com- 

 parable to oxidation, led us to experiment with hydrogen peroxide. 

 Fair growths of diatom could be obtained in sea-water prepared in 

 the manner described, but they always showed a tendency to rather 

 rapid exhaustion. As in the case of animal-charcoal water, tank- 

 water proved a much better basis for treatment with HgO^ than outside 

 water. 



Reviving Exlumded Cultures. Several experiments were carried out 

 with water from old, exhausted cultures. The sediment was filtered 



* Against this view would seem to be tlie fact, that when powdered cocoanut charcoal, 

 which has a still higher power of occluding gases, was used in the place of animal char- 

 coal, very poor cultures were obtained. 



