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.^ 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. 
