214 ELEMENTARY CHEMICAL MICROSCOPY 



reading. Make several observations of the diameter of the 

 sphere. Then illuminate the sphere by oblique light by swinging 

 the mirror far to one side; determine the diameter again, making 

 not less than three observations; the results should be the same 

 as the measurements made with axial light. Average the results. 

 The weight of the bead may now be calculated from the equa- 

 tion W = (d z X 0.5236) s where d is the diameter of the sphere 

 and s the specific gravity of the metal. 1 



For the quantitative determination of minute particles of mer- 

 cury micrometric measurements of the diameters of the globules 

 of the metal and calculations of weight therefrom are also un- 

 questionably one of the oldest and best methods at our disposal 

 in toxicological examinations, in the analysis of mineral waters, 

 urine, gases carrying mercury vapors, etc. 



Raaschou 2 has recently worked out in great detail the methods 

 -and conditions essential for the quantitative separation of 

 minute amounts of mercury from liquids. For details, the 

 student should consult the original article. 3 When dealing 

 ; with sublimates of metallic mercury consisting of so great 

 a number of tiny globules as to render measurements of the 

 diameters of all the globules impracticable, cause them to 

 unite into a few large spheres by stirring the film with a fine 

 needle, or stiff hair, or glass rod drawn down to a hair, but if 

 this is done the needle or hair must always be examined with 

 the microscope to see that no mercury has been removed by 

 clinging to the stirrer. In order that accurate measurements 

 may be made it is essential that the globules of metallic mercury 

 shall never be so large that they become flattened and thus not 

 perfect spheres. In determining the diameter of the spheres 

 proceed exactly as described above, always making several 

 measurements of the sphere diameters. From the average of 

 the data thus obtained, calculate the weight W = (d 3 X 0.5236) 

 X 13.59- 



1 For gold, 5 = 19.33; silver = 10.4; platinum = 21.15; l ea d = 11-36; mer- 

 cury = 13.59. 



2 Raaschou, Zeit. anal. Chem., 49 (1910), 172. 



3 See also page 319, Microchemical Detection of Mercury. 



