70 BIJLLETEN" 18 4, UNITED STATES NATIOiNIAL MUSEUM 



droplets found in light plessite fields. The development of such 

 very minute spheroids is exceptionally perfect in certain fields in an 

 unidentified iron illustrated in plate 74. The larger spheroids, first 

 referred to, many of which show incompletely transformed cores, 

 are evidently the product of slower growth. 



Both spheroidized taenite and the lamellar form resembling the 

 structure of pearlite are obviously due to a rate of cooling too rapid 

 to permit coalesceoce. Thus in the unidentified iron shown in plate 

 74, which bears evidence of strong reheating, owing to the rapidity 

 of cooling the taenite was entrapped in the interior of the large kama- 

 cite grains, where it coalesced into minute spheroids instead of mi- 

 grating to the grain boundaries. In the same iron, in a lamella of 

 dense plessite or darkened ("spotted") taenite, in which the gamma 

 phase was much more abundant, it segregated in cooling partly in 

 spheroids and particles, and partly in a pearlitic pattern of lamellae, 

 instead of migrating to the edges to form the usual border. In the 

 other three irons mentioned, the plessite fields show a limited devel- 

 opment of the pearlitic pattern, in addition to the larger areas of 

 spheroids. 



XIII. HEAT ALTERATIONS 



Sorby (1887) early pointed out that structm-al changes were 

 produced in octahedral irons by reheating, and he suggested a clas- 

 sification of meteoric irons according to whether they had been 

 reheated but did not indicate the nature of such heating. Cohen 

 (1905) mentioned the observations of Sorby and of Brezina and dis- 

 cussed briefly the newly expressed theory of Berwerth (1904) that 

 ataxites in general are derivatives of octahedral irons through re- 

 heating. This theory Cohen did not accept, aptly pointing out that 

 many ataxites have compositions unknown among octahedral irons. 



A decade later central illumination had come into use in the study 

 of meteoric irons, and some of its important revelations were em- 

 bodied in publications by Pfann (1917) and Bei-werth (1918) as the 

 result of investigations begun by them jointly in 1914. Pfann, 

 disregarding the factor of reheating, built up convincingly a con- 

 tinuous series of structures in accordance with the iron-nickel dia- 

 gram as then accepted. Berwerth's memoir is confined to the inter- 

 pretation of illustrations of 14 ataxites. Eight of these he treated 

 as natural products, in substantial accordance with Pfann's views; 

 the other six he classified as products of artificial heating. 



Earlier researches by Berwerth (1905) were noteworthy as estab- 

 lishing the fact that the appearance of the kamacite in octahedral 

 bands is markedly altered by heating, though he found the bounding 

 taenite unchanged. He apparently did not study the effects upon 



