CHAPTER VIII. 

 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 of materials 

 which cannot be chemically analyzed. As typical examples of 

 these cases may be cited, mixtures of starches, meals, adulter- 

 ated 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, powdered ores, powdered 

 materials of all kinds, explosives, etc., etc. 



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

 possible methods of procedure. That these methods may be 

 sufficiently accurate for our purpose the following requirements 

 must be met. The components of the mixture must differ suf- 

 ficiently in their appearance under the microscope to permit 

 their easy recognition, or they must be readily differentiated by 

 their different behaviors towards stains or reagents; the com- 

 ponents 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 a cover-glass; if of different 

 specific gravities, their specific gravities must be known. 



Most of these approximate quantitative microscopic methods 

 are based upon the fact that in normal powdered materials 

 such as meals, ground spices, powdered drugs, etc., in fact all 

 vegetable tissues and most powdered material of fairly definite 

 composition, characteristic elements are present in numbers 

 which bear to each other ratios which vary between fairly narrow 

 limits. These ratios having been first ascertained through the 

 examination of material known to be normal or of known com- 

 position, any variation in the ratios thereform is to be inter- 



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