212 ELEMENTARY CHEMICAL MICROSCOPY 



or an eyepiece with a central diaphragm opening adjusted to any 

 convenient area on the slide. Results may be expressed either 

 in numbers per cubic centimeter or in per cent by the plotting 

 method described above. 1 



In the biological examination of water the microscopic organ- 

 isms are concentrated into a few cubic centimeters of water by a 

 small sand filter contained in the stem of a funnel of special 

 design. The sand, together with the supernatant small volume 

 of water, is emptied into a test tube, given a rotary motion and 

 as soon as the heavy sand subsides, the water containing the 

 organisms in suspension is poured off and one cubic centimeter 

 transferred to the counting cell. 2 Although used primarily for 

 the purpose stated, this counting cell and method can be applied 

 to many problems involving chemical analyses. 



In order to facilitate the counting and recording of the sus- 

 pended matter found in water, Whipple has devised an eye- 

 piece micrometer with special ruling. This type of micrometer 

 has been found desirable as an aid in recording the size and 

 number of masses of amorphous matter in water. By common 

 consent American analysts have agreed to express these values 

 in terms of the areas covered by the masses found in the cell. 

 The unit employed is a square, 20 microns on a side, and there- 

 fore equal to 400 square microns; this is known as a " standard 

 unit." The eyepiece micrometer is ruled and so adjusted that 

 with a given objective and eyepiece the smallest squares are 

 equal to a standard unit, Fig. 115. 



Method II. When isolated particles of sufficiently definite 

 shape can be found and are of known composition and density, 

 it is possible to calculate their weight from micrometric measure- 

 ments. 



1 For further applications of Method I, see Meyer, Zeit. Nahr. u. Genuss, 17 

 (1909) 497: Ezendam, Zeit. Nahr. u. Genuss., 18 (1909) 462. Analysis of Starch 

 Mixtures. 



Young, Bull, no, Bureau Chem., U. S. Dept. Agric.; Pollen in Honey. 



Boedemann, Landw. Vers. Sta., 75, 134; Smut Spores in Flour, etc. 



Oerum, Biochem. Zeit., 35 (1912), 18; Fat in Milk: Vauflart, Ann. Falsif., 4 

 (1911) 381; Analysis of Meals. 



2 For details and precautions in water examination, the student should consult 

 Whipple, The Microscopy of Drinking Water. New York, Wiley & Sons, Inc. 



