AUTIFICIAL CULTURE OP MARINE PLANKTON ORGANISMS. 375 



and is sold in two g'i*ades known as "pure" and "commercial." 

 Our earlier experiments were all made with "pure" animal 

 charcoal, but subsequently the " commercial " animal char- 

 coal was largely used, and appears to give equally good, if not 

 better results. In both cases the animal charcoal is used in the 

 powdered form. Animal-charcoal water is prepared as follows : 



(1) A quantity of sea-water is sterilised by heating it in 

 a flask to 70° C, at which temperature it should be kept for 

 about twenty minutes. At the same time some animal 

 charcoal is heated sufficiently to sterilise but not to burn it, 

 covered over, and allowed to cool. When both are quite 

 cold the charcoal is added to the water (ca. 15 grm. to 

 1000 c.c), and well shaken up in it several times. After 

 an interval of half an hour or more the water is filtered 

 through fine filter-cloth,^ the whole filter having been first 

 sterilised with boilinu: sea-water, and is received in a sterile 

 flask. It is then ready for use. 



(2) For many experiments, where larger quantities of water 

 were required, the sea-water was not sterilised before being 

 treated with animal charcoal. In this case, it" the first part 

 of the filtrate be rejected, the subsequent water will generally 

 be practically sterile, and few, if any, extraneous organisms 

 will develop in it. 



(3) At a later date an automatic apparatus was set up in 

 the Plymouth Laboratory, by which very considerable quanti- 

 ties of sea- water could be treated with animal charcoal, and 

 subsequently filtered through a "Berkefeld" filter; water 

 treated in this manner we call "Berkefeld water." Tank- 

 water Avas always used in this apparatus, and was mixed with 

 animal charcoal," in a clean sulphuric acid carboy, by Mowing 

 air through Avith a pair of bellows. The mixture was allowed 

 to settle for at least twenty-four hours, and then syphoned 



' The filter-cloth iised for this jnirpose is the same as is made for use 

 in filter presses, and is known as extra-super swansdown. To prevent 

 this becoming clogged another cloth, known as hydraulic twill, was, as 

 a rule, used over it. 



" Ca. 300 grm. to 20 litres o£ water. 



