212 ELEMENTARY CHEMICAL MICROSCOPY 



size by the number of fragments and the specific gravity of the 

 fiber. 



Quantitative microchemical methods with reference to the 

 handling of minute amounts of material and weighing on a 

 Nernst micro balance; the titration of tiny volumes of liquid; 

 the measurement of tiny volumes of gas, etc., which do not 

 require the application of the microscope need no discussion here, 

 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 photogrpah 

 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 



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



Pregl: Quantitative organische Mikroanalyse, Berlin, 191 7. 



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

 Petrographic Methods, p. 290. See also Coghill and Bonardi: Quantitative 

 Microscopy of Pulverized Ores, Tech. Paper 211, Bur. Mines. 



3 Eng. Record, Mar. 191 5. 



