QUANTITATIVE ANALYSIS BY MEANS OF THE MICROSCOPE 2OQ 



is present in the proportion of 3 to 7 per cent the results found 

 are very close to the actual per cent, but when 7 per cent is 

 reached, the beginner has trouble in obtaining reliable counts, 

 and above 10 per cent the method requires great manipulative 

 skill. 



It must, however, be borne in mind that a method of this 

 sort even at its best gives merely a close approximation to the 

 true value. 



The chief difficulties which will be encountered are those of 

 removing equal amounts in every case upon the end of a tiny 

 spatula; of obtaining a uniform distribution of the material 

 throughout the drop; and of lowering the cover glass upon the 

 preparations without destroying the uniformity of distribution 

 of particles or introducing air bubbles. A little practice, how- 

 ever, will enable the analyst to work rapidly and accurately. 



If more nearly accurate sampling is desirable, a portion of 

 the material is carefully weighed out, spread on a piece of glass 

 or glazed paper in a thin square of as nearly uniform thickness 

 as possible and then sampled by "quartering" in the usual 

 manner 1 until a section equivalent to 2 to 4 milligrams is obtained 

 for transfer to the object slide. 



An even better method consists in carefully weighing out a 

 small portion of the material to be examined and mixing it with 

 a known weight, several times greater, of a finely and uniformly 

 powdered substance very soluble in water (or other solvent). 

 After thorough mixing, a small portion of the preparation is 

 removed, accurately weighed and transferred to an object slide. 

 The selected mounting liquid is added, causing the soluble 

 diluting solid to dissolve and disappear, leaving a known weight 

 of the insoluble material under investigation evenly spread upon 

 the slide. The number of foreign particles in this tiny portion 

 can then be counted. In the case of most food products, such 

 as starches, flour, meals, spices, etc., powdered sucrose, dextrose, 

 lactose or soluble dextrine are most useful as diluents. 



When the mixtures under examination are of a density only 

 very slightly greater than water and are insoluble therein, and 

 1 Kraetner, J. Am. Chem. Soc., 21 (1899), 659. 



