QUANTITATIVE ANALYSIS BY MEANS OF THE MICROSCOPE 199 



preted as evidence that the material in question is abnormal, 

 of inferior (or superior) grade or sophisticated. From the mag- 

 nitude of the variation of the ratio found from that in the stand- 

 ard or from the standard unit used, the percentage composition 

 of the powdered material may be calculated. 1 



In microscopic quantitative analyses we may (i) ascertain 

 the ratios to each other of the different components present and 

 compare these ratios with those obtained on known standards; 

 or (2) compare preparations made from the material of unknown 

 percentage composition with preparations containing the same 

 components in known amounts, the standards used in the com- 

 parison having been carefully prepared in the laboratory; or 

 (3) we may, by micrometric measurements compute the areas 

 (or employ a planimeter) and thus obtain a clue to the percentage 

 composition since volume per cents are to each other as the areas, 

 and from the volume per cents weight per cents may be computed 

 if the specific gravities of the components are known: these 

 relations can be ascertained as described below; or (4) in the 

 case of mixtures solidifying from fusion where the melt on 

 freezing has been found to give rise to phases sufficiently char- 

 acteristic in appearance yet differing according to the percentage 

 composition, the recognition of these crystalline phases will 

 serve to indicate the probable composition of the mass. 



The last method (4) is restricted to materials such as alloys 

 or related substances. An expert, knowing the characteristic 

 appearance following certain treatments, is able, on studying 

 materials of known components but of unknown percentage, to 

 decide upon the probable proportion of the chief constituents 

 without the necessity of a quantitative analysis. This type of 

 analysis by means of the microscope can be practiced only by 

 experts after long study and investigation and cannot therefore 

 be here discussed. 



1 A thorough discussion of this sort of microscopic quantitative analysis with 

 many illustrative examples will be found in: Schneider: Microbiology and Micro- 

 analysis of Foods, p. 92. Blakiston's Son & Co., Philadelphia, 1920. Or in: 

 " Microanalysis of Powdered Vegetable Drugs," by the same author. Second 

 Ed. p. 141. Blakiston's Son & Co., 1920. 



