METHODS OF PLANKTON RESEARCH. 517 



alcohol, the volume will be quite inaccurate, because a 

 precipitate forms from the salt water that has been round 

 the organisms when first fixed. This in appearance is 

 like a diatom deposit, and the volume of a catch may be 

 reduced to one-third by transferring" from alcohol to 

 water, owing to the removal of this precipitate. Ordinary 

 measuring glasses are of no use for measuring accurately 

 small catches. A special make of glass tube is used, the 

 bottom of which is drawn out into a cone ending in a 

 blunt point, so that a small volume of catch will occupy 

 a considerable depth of this narrow termination. The 

 plankton catches in distilled water are transferred to these 

 tubes and allowed to settle for 24 hours. A mark is then 

 made with ink on the outside of the tube at the level to 

 which the sediment attains, and the catch is again 

 removed. The quantity of water measured out by means 

 of a burette, which takes up the same space as the 

 sediment, will be the volume of the catch. 



This volume estimation is necessarily very rough, 

 since, especially if diatoms be present, a quantity of 

 liquid remains between the organisms and causes the sedi- 

 ment to appear much greater in volume than it really is. 



Having found the volume of the plankton in cubic 

 centimetres, it is multiplied by 80 (for the Middle Apstein 

 Net pulled up j metre in 1 second), and this gives the 

 volume present in a column whose area is one square 

 metre and whose length is the distance through which 

 the net has been hauled. For purposes of comparison 

 and the making of curves, the average volume per cubic 

 metre is generally reckoned from the above. 



The next division of the work consists of counting 

 the organisms. For general use with the Middle Net 

 the catch is brought into 50 c.cm. of distilled water. If 

 the catch is very large a further dilution may be ueees- 





