from individual particles are no longer observed when the powder is examined 

 in a bright light while still in the mortar. If the material does not grind readily, 

 it may be filed with a clean single-cut fine-tooth file, using no more pressure 

 than is absolutely essential. If the specimen must be preserved in its original 

 form, the specimen can be mounted in a suitable rotating or oscillating device 

 in such a manner that the sample-to -film distance remains constant. 



Before the method of mounting the powdered specimen is selected, the 

 optimum thickness of the sample to be used should be determined. The proper 

 thickness can be calculated if sufficient information is available concerning the 

 specimen. Otherwise, the optimum thickness usually can be estimated approxi- 

 mately by an experienced operator from the amount of the undiffracted X-ray 

 beam that penetrates trial specimens, as determined with a fluorescent screen. 

 This thickness can be calculated from the equation 



2 



where /x is the linear absorption coefficient calculated from the mass absorption 

 coefficient according to the relationship; 





d being the density of the material, p the elemental fraction in the compound and 

 — - — the mass absorption coefficients of the elements for the wavelength of the 

 radiation used. The values for — ^ — can be found in table form in volume 2 



r 



of "International Tabellen zur Bestimming von Kristallstrukturen" (Cullity, 

 1956 or Barrett, 1952) . 



For materials of high atomic weight the optimum thickness may be so 

 small as to necessitate dilution of the crystalline material with amorphous 

 diluents such as flour, cornstarch, or gum tragacanth (Davey, 1934; or American 

 Society for Testing Materials, designation E43-42T, 1942) . In any case, however, 

 these diluents should be avoided or kept to a minimum since some of them 

 (e.g., raw cornstarch) produce a crystalline pattern of their own, or an amor- 

 phous pattern with very broad lines (halos). These superposed patterns of the 

 diluents often cause a considerably localized background fog, with consequent 

 difficulty in observing lines in the regions of the amorphous bands. 



Some of the early investigators recommend that the ground and diluted 

 samples be packed into capillary tubes with an inside diameter of 0.4 to 0.6 mm 



156 



