GERMAN PLANKTON INVESTIGATIONS. 



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by the third method, that of chemical analysis. The 

 weight estimation, when practicable, gives ns the best 

 estimation of mass. It has, however, one decided dis- 

 advantage, and that is any material investigated in this 

 way is invariably destroyed, so that it cannot be employed 

 in cases where it is desired to preserve the material, as in 

 the case of oceanic expeditions which have been fitted 

 out at great cost. 



The third method, that of chemical analysis has been 

 developed by Brandt (7). The method was also employed 

 by Hensen in his first plankton estimation work. The 

 results of these two methods are described further on. 

 The methods are briefly : 



1. The weight in the damp condition, in addition to the 



volume of the mass after settling down. 



2. The dry weight. 



3. The weight of ash. The difference between 2 and 3 



gives the weight of dry organic substance. 



4. The percentage of silica. Obtained by weighing the 



residue insoluble in water and acids. 



These methods may either be employed for the whole 

 of the catch or for certain constituents of the same as, for 

 example, for Copepoda or Ceratium. 



For chemical investigation only fresh material or 

 material preserved in alcohol can be investigated. By 

 the use of all other preservatives either some of the sub- 

 stance of the catch becomes lost or destroyed or else some 

 of the preservative medium remains behind and vitiates 

 the results. For killing and preservation pure rectified 

 spirit (70 per cent.) was found most suitable, and for the 

 preservation, bottles and flasks with glass stoppers must be 

 used. If it is wished to analyse a separate portion of Hie 

 catch, the separation of such portion must be made before 

 the organisms are killed. This can be done by the use 



