CH. Vl] 



QUANTITATIVE PLANKTON INVESTIGATIONS 



133 



the coDtents are uniform in composition, but with a mixture in 

 -which the constituents ta be estimated are particles in suspen- 

 sion and quickly settle down when it is left at rest. We cannot 

 therefore use the ordinary pipette of the chemist to remove the 

 fractional part. Hensen had to devise the " Stempel-Pipette " for 

 this purpose. \ This instrument is shewn in Fig. 27. It consists 

 of a strong glass flask which is closed by a bung 

 through which passes a wide glass tube T in which 

 there is a piston. The lower end of this piston P is 

 a solid piece of metal which slides accurately up 

 and dow^n the glass tube but without much friction. 

 Round the sides of the piston the metal has been cut 

 aw^ay so that the volume of the cut-out portion is 

 exactly 1 c.c, or any other convenient volume. The 

 calibration of the piston is effected by tilling the 

 cavity with mercury and w^eighing the latter, 

 and then cutting away more metal until it is found 

 that the weight of the mercury contained in the 

 cavity is exactly that of 1 c.c. of the latter metal. 

 The diluted catch is now brought into the flask 

 and the latter is filled with water up to the mark (which may 

 indicate a volume of 100, 250, 1000 or any other number of c.c), 

 and the whole is thoroughly shaken up so as to produce an uniform 

 mixture. Then before the organisms settle the piston is suddenly 

 drawn up and the glass tube is removed from the flask. 



The cubic centimetre of liquid containing the organisms to be 

 counted is now brought on to a glass plate which is ruled into 

 squares, and this plate is put on the stage of a microscope which 

 has a mechanical movement which enables each of these squares 

 to be brought successively beneath the lens, and ensures that every 

 square on the plate will come under observation. Square after 

 square is then passed under the lens and the organisms on it are 

 counted. Suppose that the catch consists of very many diatoms 

 and of few copepods. Hensen had a box in front of him while 

 he was examining the catch, and this box was divided into as many 

 compartments as there were species to be counted. As each 

 square was examined as many counters were dropped into the 

 specific boxes as there were organisms on it. Hensen appears to 



Fig. 27. 

 The Stempel- 

 Pipette 

 (in section). 



