194 ELEMENTARY CHEMICAL MICROSCOPY 



crystals are checked as counted by means of a pencil made for 

 writing on glass. 



The magnifications recommended by the American Society 

 for Testing Materials are as follows according to the character 

 of the specimen: 



Non-ferrous alloys. .25, 75, 150 or 250 diameters. 

 Steels 50, 100, 250 or 500 diameters. 



To obtain the number of grains per square millimeter, the 



total number of grains found is multiplied by a factor depend- 



M 2 

 mg upon the magnification used. This factor = / = - — , 



5000 



where M is the magnification employed. If instead of express- 

 ing the number of grains per square millimeter it is desired to 

 obtain the average diameter of the crystals in millimeters or 

 to learn their average areas in square microns the following 

 formulas may be employed: 



;/ = fx; 



d = 



a = 



1 



vV 

 1 ,000,000 



H 



x = total number of grains found; 



M 2 



f = factor for magnification,/ = ; 



5000 



n = number of grains per square millimeter; 



d = diameter of average grain counted expressed in mm.; 



a = area of average grain in yi 2 . 



Calibration of Sieves. — Sieves to be used in accurate analysis 

 in grading or classifying finely divided material must have the 

 wire cloth carefully made and applied to the metal frames. 

 Carelessness in weaving or in stretching the cloth too tightly 

 upon the frames will give rise to irregular openings and the sieve 

 becomes thereby unreliable and useless for the purposes to 

 which it is to be put. The U. S. Bureau of Standards has issued 



