ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 511 



has an appreciable carbon content, which increases as the total carbon 

 increases, and that eutectoid ferrite contains in solution • 06 to • 14 p.c. 

 carbon according to the temperature. 



Malleable Cast-iron.* — 0. W. Storey has made a microscopical 

 study of the changes which occur in the annealing process in the 

 manufacture of malleable castings. Specimens of white iron containing 

 2*6 p.c. carbon, all combined, were heated under different conditions, 

 the factors that were varied in different series of experiments being : 

 (1) maximum temperature ; (2) length of time for which the maximum 

 temperature was maintained ; (3) rate of cooling ; and (4) packing 

 material. The nature of the packing material did not affect the 

 structure of the interior of the annealed casting, but had some influence 

 on the structure of the skin. In general, the outside layer consisted of 

 ferrite when the conditions of annealing were distinctly oxidizing, while 

 under less oxidizing conditions a steely outside layer containing pearlite 

 resulted. The interior of a well annealed " black-heart " casting consists 

 of ferrite and temper-carbon. Slow rates of cooling are necessary to 

 produce this, and pearlite is formed if the cooling is too rapid. Anneal- 

 ing temperatures exceeding 800° C. are necessary to cause the cementite 

 to decompose within a reasonable time. The higher the annealing 

 temperature the shorter is the time required. The - author suggests that 

 the cementite decomposes while it is in solution in the iron, that free 

 cementite does not decompose, and that consequently no decomposition 

 of cementite occurs below Ar x . 



Diffusion in Solids.f — L. Cluillet and V. Bernard describe their 

 experiments on " reserves " in cementation, which have led to the study 

 of the more general question of diffusion in solid metals. In the 

 operation of case-hardening it is frequently necessary to prevent the 

 cementation of portions of the surface of the article undergoing treat- 

 ment, since the hardened skin is desired only on the remainder of the 

 surface. Protection of the surfaces which are to remain soft is secured 

 by covering them with substances calculated to prevent the access of 

 the solid or gaseous cementation medium. Clay and similar materials 

 are only partially effective, since they are porous at case-hardening 

 temperatures and permit the access of carbon monoxide to the coated 

 surface. The effectiveness of any coating is most readily determined by 

 microscopic examination of a section through the coated surface after the 

 case-hardening operation. Copper deposited electrolytically or by the 

 Schoop spraying process, was found to be effective if the layer was 

 sufficiently thick. Nickel did not prevent cementation, unless used in 

 very thick layers, apparently because nickel is permeable to carbon 

 monoxide. In the experimental study of diffusion in solids, specimens 

 in which two metals were in perfect contact were prepared by mechanical 

 means or by electrolytic deposition , and were heated as shown in the 

 table below. In each case the temperature of heating was below the 

 melting point of either metal, and was in a range in which a solid solu- 



* Met. and Chem. Eng., xii. (1914) pp. 383-9 (12 figs.). 

 t Rev. Metallurgie, xi. (1914) pp. 752-65 (32 figs.). 



