QUANTITATIVE ANALYSIS BY MEANS OF THE MICROSCOPE 203 



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. 



To facilitate the taking of samples of uniform size of starch 

 mixtures or other very fine powders which are easily compacted, 

 brass or glass rods, with tiny spherical depressions (1.5 mm. in 

 diameter, 0.5 mm. deep) in their ends, may be advantageously 

 employed. 1 The material to be analyzed is spread in a thin 

 layer upon a piece of glass, the sampling rod is pressed gently 

 upon the layer, the depression in the end of the rod is thus filled. 

 The rod is lifted off and the end wiped gently with a finger tip, 

 care being taken to avoid displacing the mixture retained in 

 the end of the rod. A light blow upon an object slide will dis- 

 lodge the pellet which can then be distributed evenly in a drop 

 of mounting medium. The " curve " for the standards must 

 be prepared with the same rod as used in sampling the unknown 

 and as nearly as possible equal pressures must be used in filling 

 the depression. 



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 man- 

 ner 2 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 



1 Communicated to the author by Dr. H. S. Booth of Western Reserve University. 



2 Kraemer, J. Am. Chem. Soc., 21 (1899), 659. 



