TEE MICROSCOPY OF ANTHRACITE COAL 309 



in the upper end. In the lower end is placed a two-hole rubber 

 stopper through which a thermometer is inserted. The temperature 

 is raised to 250° Centigrade by heating the tube with a Bunsen 

 burner just beyond the coal, between it and the intake. The action 

 is continued until the polish begins to grow dim. Great care must 

 be exercised to prevent the surface of the coal from coking. The 

 forms revealed by this method of treatment are not as sharply 

 defined as those produced by direct heating and since it requires 

 more time, care, and apparatus, it is not recommended where 

 direct heating can be used. The only advantage is that the sur- 

 face remains fiat and therefore shows some structures in coal which 

 might otherwise be largely reduced to fragments by direct heating. 



The etched coal is finally studied with the metallographic 

 microscope, using vertical illumination from a carbon arc. A good 

 working objective is one of medium power giving a magnification 

 of about 250 diameters. A clean-cut image can be obtained up to a 

 magnification of 1,000 diameters. In all cases the eye can discern 

 finer details than are shown in the photographs. 



The method as outlined, except for a modification of the etching 

 process, has been successfully applied to bituminous coal and canuel 

 coal. The polished cannel coal was held for a few moments in the 

 oxidizing Bunsen flame without being previously heated in the 

 oven. Good results can probably be obtained with carbonaceous 

 shale and other materials of similar nature. 



GENERAL RESULTS 



A preliminary examination of the specimens of anthracite from 

 the different fields shows that all varieties are composed of laminae 

 of different luster, texture, and thickness. Brilliant jet black layers 

 alternate with glossy black or gray bands in which are imbedded 

 thin sheets of dull material resembling charcoal. The brilliant 

 black layers vary in thickness from a few microns to many centi- 

 meters. The glossy gray bands are thicker than the jet black ones 

 although, on close inspection, they are shown to contain many thin 

 sheets of the brilliant black coal and thin sheets of dull material. 

 The brilliant layers are more compact than the duller ones and 



