2 BU'LLEl'INT IS 4, UNITED STATES NATIONlAl, MUSEUM 



rich domain, the earher investigations were accurate and exhaustive 

 and still form an indispensable basis for the study of meteoric iron. 



A great many of the older descriptions are now of little value 

 beyond the fact that they give the history and general character- 

 istics of specimens. Octahedrites were easily recognized and classi- 

 fied, but scant information could be given regarding hexahedrites, 

 and as to ataxites none at all. A restudy of all irons not recently 

 described would be of great interest and doubtless would reveal 

 errors in classification. 



As mentioned later, the technique of analysis has so greatly im- 

 proved that many new analyses should be made, especially of irons 

 whose structure makes the proportion of certain components impor- 

 tant. In many cases the only published analyses are 50 to 100 

 years old, and many of these seem inconsistent with the structure 

 of the irons. 



Macroexaminatioii ; etching. — Visual investigation of the structure 

 of metals is based upon the appearance of a polished surface, on 

 which structural components of varying solubility become visible 

 when treated with an etching reagent. Etching may be of the 

 ordinary kind, termed macroetching, or metallographic (micro) 

 etching. 



For the former, which is used for museum specimens, the surface 

 is given a reasonably good polish and then etched with a dilute acid 

 until its macrostructure has been brought out as clearly as possible. 

 In the class of irons known as octahedrites the Widmanstatten pat- 

 tern is revealed, often strikingly. In certain other types, which 

 may possess a wealth of microscopic details, a mat surface is produced 

 with no visible structure. Even under the microscope the rough- 

 ness of the etched surface makes fine details invisible. 



With macroetching the larger structural features can be studied 

 well with an ordinary microscope at low magnification in oblique 

 light — such as the general character of the Widmanstatten struc- 

 ture, the presence of small particles, and the size and form of larger 

 inclusions. The compilation of photographs of meteoric irons pub- 

 lished by Brezina and Cohen (1887), made in this way at low or 

 moderate magnification, was an invaluable contribution to science 

 and is still useful. But with macroetching it is quite impossible to 

 study such features as granulation, minute inclusions, details of 

 Neumann lines, and eutectoid structures or to observe the very 

 important effects of selective etching. 



Microexamination. — For metallographic study the procedure and 

 the results are quite different. The polish, instead of being only 

 reasonably good, must be of the utmost refinement — so perfect that 

 no imperfections are visible even under high magnification. The 

 surface is then etched very lightly — so lightly that the appearance 



