200 
ELEMENTARY CHEMICAL MICROSCOPY 
The second method may he employed in the quantitative 
analysis of all mixtures consisting of individual particles, frag¬ 
ments or crystals, which are not too large for microscopic exami¬ 
nation, providing the component particles differ sufficiently in 
appearance to permit .of identification and that mixtures of 
known percentage composition can be prepared in the laboratory. 
Since this method has its chief application in estimating the 
amount of adulteration in a substance, the discussion will be 
confined to this aspect only. 
Method. — Prepare three standard mixtures containing the 
same components as the commercial products to be examined. 
In preparing these standards the adulterant must be carefully 
weighed out and added to a definite weight of the pure product; 
after thorough mixing, three mixtures of known per cent of adul¬ 
teration are thus obtained. 
From each one of these standards in turn, several like portions 
are taken, placed upon glass object slides in a drop or two of 
suitable medium (usually glycerine and water i : i),’ distributed 
uniformly in the mounting medium and covered with a square 
cover-glass, care being taken to avoid air bubbles; use just 
sufficient mounting medium to ensure an even distribution of the 
material throughout the whole area covered by the cover-glass 
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 
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 
a before. Upon a sheet of ‘‘ coordinate ” paper lay out per 
cents of adulteration as ordinates and numbers of foreign par- 
1 Smith, Health Mag., 6 (1898), 286, has shown that in the case of starch mix¬ 
tures a mounting medium of equal parts of glycerine, water and 50 per cent acetic 
acid is preferable. 
