THE METALLOGKIAPHY OF METEORIC IROOST 97 



The foregoing is only a general outline of the technique of polish- 

 ing, which varies in practice according to individual preferences. 

 Further information can be found in textbooks on metallography, 

 and instruction by someone skilled in such work is still more helpful. 

 Microetching. — Before the surface is microetched it should be 

 examined under the microscope for nonmetallic inclusions — such as 

 graphite, silicates, and oxides — that are visible without etching. 

 Schreibersite and cohenite, though showing no contrast in color, are 

 usually visible because of their being slightly in relief. The reve- 

 lation of such structures in this way without etching, because of 

 variations in hardness, is sometimes referred to somewhat inaptly 

 as "relief etching." 



The etchant is best applied with absorbent cotton gently wiped 

 over the whole surface; the progress of the attack should be closely 

 watched, as it is much faster on some irons than on others. Pre- 

 viously the specimen should have been washed with benzole or 

 acetone, applied with cotton, to remove any trace of grease in case 

 the hard form of jeweler's rouge was used in polishing. If the pow- 

 dered form was used, washing in water is sufficient. When etched 

 the surface should be thoroughly washed in a stream of water, dried 

 before a fan or air blast, and placed in a desiccator. 



The time of etching depends upon the etchant and upon the mag- 

 nification to be used. For most meteoric irons the time of etching 

 is short, especially for examination or photography at high magni- 

 fication. With picral etching, which the author has found most 

 generally useful, the time varies from 3 to 10 seconds for high magni- 

 fication (1,000 or more). It may be from 30 to 80 seconds, some- 

 times more, for work at 50 diameters. These limits are only approx- 

 imate, varying according to composition and structure. 



A convenient procedure is first to give the specimen a very fight 

 etch for work at high magnification; then somewhat more for pho- 

 tography at 250 to 500 diameters; and finally for moderate or low 

 magnification a still stronger one. If the stronger etch is given first, 

 it is necessary to repolish the specimen for the light etch required 

 for high magnification. It should always be borne in mind that 

 the results of etching, even with an apparently uniform technique, 

 are not invariable; so when peculiar or unexpected effects are noted, 

 the process should always be repeated to guard against unsound 

 conclusions. 



Etching reagents. — In the study of artificial irons a great variety 

 of etchants have been used, which may be found in textbooks. With 

 meteoric irons, however, the following list includes aU that would 

 ordinarily be employed: 



Picral (5 percent picric acid in alcohol) and nital (5 percent nitric 



