QUANTITATIVE ANALYSIS BY MEANS OF THE MICROSCOPE 207 



and to completely fill the space below the confining cover yet not 

 have a loss by the squeezing out of the liquid. One of the prepara- 

 tions is then placed upon the stage of the microscope, and a count 

 is made of the number of particles of the adulterant which are 

 found in a field of the microscope. Having counted the foreign 

 particles in several different fields, a second preparation from 

 the same mixture is tried and so on until at least twenty or more 



q 

 5 



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Average number of foreign particles per field 



FIG. 112. 



counts have been made. A different mixture is then taken and 

 the number of foreign particles determined exactly as in the first. 

 Finally, the third known mixture is examined and counts made 

 as before. Upon a sheet of "coordinate" paper lay out per 

 cents of adulteration as ordinates and numbers of foreign par- 

 ticles as abscissas. The averages of the counts of these particles 

 obtained in each of the three mixtures of known per cent adul- 

 teration are then marked upon the coordinate paper in their 



