64 Micro-Metallography of Iron. 



apparently further broken up or subdivided into a considerable 

 number of smaller crystals, inclosed within the boundary or peri- 

 phery of the primary crystals. 



In the coarse of further experiments on the cooling of large masses 

 of wrought iron, the author has also found, by the use of high power 

 objectives, that the secondary crystals sometimes inclosed a still more 

 minute form of crystals of pure iron, of the cubical form, which mav 

 hence be regarded as constituting a tertiary system of crystallisation 

 in pure metallic iron. These experiments therefore indicate that large 

 masses of heated wrought iron, on cooling from above the temperature 

 of the crystallisation of metallic iron, viz., 740 C..* are capable of 

 crystallising in three distinct modifications, which may tentatively be 

 called the primary, secondary, and tertiary system of crystallisation 

 in iron, these various crystalline modifications being all, however, 

 connected with the regular system of crystallisation. The author 

 has microscopically examined numerous large masses of practically 

 pure wrought iron varying in weight from about 2 tons to 4 cwt., 

 and even less, and he finds these subcrystalline formations to be 

 frequently present, consequent on the slow cooling of such large 

 masses. 



The crystals of this secondary formation are not often distinctly 

 discernible in smaller masses of metallic iron, such as rolled rods, 

 plates, or sheets, as these in the course of manufacture rapidly cool, 

 and are frequently manipulated during the finishing processes at tem- 

 peratures below the crystallising point of wrought iron (740 C.). 

 The author has, however, observed the presence of this subcrystalline 

 structure in small masses of iron, but, in these instances, the sub- 

 crystals are generally smaller in size and not always so distinctly 

 marked as those found in larger masses of metallic iron. 



The microscopical examinations were made on carefully-prepared 

 and polished samples, etched in nitric acid (1 part HN0 3 , sp.gr. 1*20, 

 nnd 49 parts water), and by the use of high microscopical powers 

 (i inch to -j 1 ^ inch, and other objectives). The drawings were accu- 

 rately made with the camera Lucida. 



In each observation the etching was prolonged, under constant 

 observation with lenses, a suitable time to develop the accurate 

 structure of the metal. 



The varied forms of crystallisation observed in this research and 

 referred to in this paper appear to be of such novel metallurgical 

 interest, that the author felt it desirable at once to record the obser- 

 vations, and he hopes to be able to furnish the results of further 

 investigations in this direction. 



* The temperature at which pure iron crystallises, riz., 740 C., has recently 

 been approximately determined with great care and accuracy by Professor J. O. 

 Arnold, F.C.S., at the Sheffield Technical School. 



