. Conditions of life in the sea; a short account of quantitative marine biological research. Marine biology. 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
. Conditions of life in the sea; a short account of quantitative marine biological research. Marine biology. 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 , 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 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 ), 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 a
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