2l6 ELEMENTARY CHEMICAL MICROSCOPY 



since we are dealing solely with the application of the micro- 

 scope to the solution of chemical problems. 1 



Volume and Weight Per Cents from Area Measurements. 

 The quantitative analysis of heterogeneous material in thin sec- 

 tions through the determination of the areas occupied by the 

 different components, as ascertained from their images when 

 seen in the microscope, has long been employed by petrologists. 



The process is briefly as follows: The outlines of the areas of 

 the component under consideration, in a given field of the micro- 

 scope, are traced upon coordinate paper by means of a drawing 

 camera; the value of a square of the paper is ascertained with 

 a stage micrometer as hereinbefore described. The areas of the 

 tracing may then be computed or may be accurately determined 

 by means of a planimeter. Or the preparation may be photo- 

 graphed with a coordinate (net-ruled) ocular in place, the value 

 of the rulings in the image ascertained in the usual manner and 

 the areas of the different component-sections in the photograph 

 computed. 2 



From the computed areas, volume per cents may be calcu- 

 lated, and knowing the specific gravities of the components, 

 weight per cents are easily ascertained. 



This method of quantitative microscopic analysis has recently 

 been applied by Johnson to the examination of concretes. He 

 has shown 3 that it is a simple matter to ascertain, whether, in a 

 given concrete structure, a contractor has complied with the 

 specifications as to proportions of sand, gravel and cement 

 and further whether the material was properly mixed and wetted. 



Estimation of Molecular Weights by Micrometric Measure- 

 ments. Barger 4 has described a most ingenious micrometric 

 method whereby the molecular weight of a substance may be 

 determined, providing a large enough amount of the material 

 for weighing upon an analytical balance is available. 



A solution is made of known weight content of the substance 



1 See Donau, Die Arbeitsmethoden der Mikrochemie, Stuttgart, 1913. 



2 For further details as to rock analysis and for bibliography see Johannsen, 

 Petrographic Methods, p. 290. 



3 Eng. Record, Mar. 1915. 



4 Barger. J. Chem. Soc. (London), 85 (1904), 286. 



