210 
ELEMENTARY CHEMICAL MICROSCOPY 
it is transferred to a drop of glycerine and water (i : i) on a 
glass object slide by picking it up with a drawn-out glass rod 
slightly moistened. Bring the metallic sphere under the center 
of the micrometer eyepiece, use an objective of low power, 
illuminate with axial light, with the Abbe condenser well lowered 
using a small diaphragm opening. Focus up slowly and as soon 
as the image reaches its maximum diameter record the scale 
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 = X 0.5236) s where d is the diameter of the sphere 
and 5 the specific gravity of the metal.^ 
[: For the quantitative determination of minute particles of 
mercury micrometric measurements of the diameters of the 
globules of the metal and calculations of weight thereform are 
also unquestionably 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 ^ has recently worked out in great detail the meth¬ 
ods and conditions essential for the quantitative separation of 
minute amounts of mercury from liquids. For details, the 
student should consult the original article."^ 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 
'For gold, 5 = 19.33; silver = 10.4; platinum = 21.15; ~ mer¬ 
cury = 13.59. 
^ Raaschou, Zeit. anal. Chem., 49 (1910), 172. 
® See also page 365, Microchemical Detection of Mercury. 
