ZOOLOGY AND BOTANY, MICROSCOPY, ETC, 637 



J. Czochralski. Three types of etching are distinguished and discussed : 

 (1) crystal-boundary etching, (2) crystal-field etching, (3) crystal-figure 

 etching, each type being illustrated by photomicrographs. Tlie effect 

 of cold-working upon structure is considered. The last part of the 

 paper is devoted to a discussion of the methods of polishing specimens, 

 and the application of different etching reagents, including heat-tinting 

 and electrolytic methods of etching. A table is included which shows 

 the reagents to be used to bring about the three types of etching for 

 most of the common metals and their alloys. 



Alloys of Iron and Boron.'' — The constitution of alloys of iron and 

 boron containing up to 11 p.c. boron have been studied by N. Tschi- 

 schewsky and A. Herdt, by taking cooling curves and by microscopic 

 examination of a series of alloys, which were prepared by melting together 

 pure Swedish iron and requisite amounts of ferro-boron containing 

 23' 7 p.c. boron. For the examination of structures the alloys were 

 slowly cooled, and etched with sodium-picrate. The polished surfaces of 

 eutectic alloys present a more " pearly " appearance than the correspond- 

 ing steels, so that the eutectic can be termed " boron pearlite." This 

 eutectic becomes perceptible at 0*08 p.c. boron, and appears as thin 

 pearlitic lines separated by masses of ferrite. As the boron-content 

 'increases, the pearlitic veins increase in width, till at 3"1 p.c. boron the 

 eutectic composition is reached. With further increase in boron-content 

 crystals of boride (FeaB) appear. The boride assumes well-defined 

 prismatic forms, and is coloured, when etched, with sodium-picrate, and 

 may be termed " boron cementite." The alloy containing 8 • 85 p.c. boron 

 consists entirely of long prismatic crystals of the boride. Polished 

 sections of alloys exceeding 9 p.c. boron were very difficult to prepare 

 owing to brittleness. The equihbrium diagram prepared from the results 

 shows the formation of a eutectic on solidification at 3" 1 p.c. boron and 

 a temperature of 1135° C. All alloys show a change in the solid at about 

 760° C., corresponding to the transformation of gamma- to alpha-iron. 



Cementation of Iron by Hydrocarbon Gases.f — F. C. Langenberg 

 describes experiments with a specially designed electric furnace in which 

 specimens of pure iron were subjected to the action of (1) illuminating 

 gas, (2) acetylene, under varying conditions with regard to temperature, 

 time, rate of flow and pressure. The furnace could be evacuated before 

 heating up, thus no oxidation of the specimens occurred, and the 

 amount of carbon absorbed was determined by noting the increase of 

 weight and the depth of the cemented case by the microscopic examina- 

 tion of transverse sections. Under certain conditions it is shown that 

 the gases actually exert a decarburizing action. This occurs with slow 

 rates of flow, and is due to deposition of carbon from the gas before 

 reaching the temperature of the hot portion of the furnace. A 

 specimen of white iron was reduced in carbon-content from 2' 5 to 

 6*5 p.c. by this action. The structure of the decarburized white 



* Rev. Soc. Rus3. M^tallurgie, i. (1915) pp. 535-46, through Rev. M^tallurgie 

 ■xiv. (1917) Extraits, pp. 21-7 (7 figs.). 



t Journ. Iron and Steel Inst., xcv. (1917, 1) pp. 129-53 (12 figs.). 



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