CHAPTER X. 

 QUANTITATIVE ANALYSIS BY MEANS OF THE MICROSCOPE. 



Some of the most difficult problems with which the chemist has 

 to deal are those requiring an opinion as to the probable per- 

 centage composition or amount of adulteration in materials which 

 cannot be chemically analyzed. As typical examples of these 

 cases may be cited, mixtures of starches, meals, adulterated 

 flours, spices, teas and other food products; mixtures in which 

 " firsts" have been sophisticated with an inferior quality of the 

 same material; adulterated pigments; mixtures of wood pulps, 

 paper pulps, textile fibers, etc. 



In the solution of problems of the above type there are several 

 possible methods of procedure, but only two need occupy our 

 attention. That these methods may be sufficiently accurate for 

 our purpose the following requirements must be met. The 

 components of the mixture must differ sufficiently in their 

 appearance under the microscope to permit their easy recogni- 

 tion, or they must be readily differentiated by their different 

 behaviors towards stains or reagents; the components must not 

 differ materially one from the other in specific gravity and must 

 be small enough in size to allow mounting on an object slide and 

 covering with an object glass; if of different specific gravities, 

 their specific gravities must be known. 



We may (i) compare preparations made from the mixture of 

 unknown percentage composition with preparations made from 

 similar mixtures of known percentage composition which have 

 been carefully prepared in the laboratory; or (2) we may by 

 micrometric measurements ascertain the volume of the com- 

 ponent whose percentage in the mixture is sought and from its 

 known density compute its weight and hence its per cent in the 

 mixture; or (3) in the case of solids made by fusion where the 

 melt on freezing has been found to give rise to phases sufficiently 



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