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 = (d 3 X 0.5236) s where d is the diameter of the sphere 

 and s the specific gravity of the metal. 1 



t 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 2 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. 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 



1 For gold, s = 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 365, Microchemical Detection of Mercury. 



