200 ELEMENTARY CHEMICAL MICROSCOPY 



The second method may be 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), 1 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. 



