ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 393 



Metallography, etc. 



Importance of Centring in Microscopic Metallography.* — L. 

 Guillet describes a stage fitting designed by Le Grrix for the purpose of 

 bringing the same field into view in successive examinations of a section. 

 The edge of the section is grooved at one point. Two small angle- 

 blocks are fixed at right angles to each other on a brass plate fitting on 

 the stage. The section is placed so that one angle-block fits into the 

 groove, while another point of the edge of the section is in contact with 

 the other block. The author describes a number of examples of photo- 

 graphs of the same field after successive etchings, in sections of steel, 

 cast iron, brass, etc. 



Constituents of Quenched Steels.f — P. Breuil reports upon the 

 research undertaken by him as the outcome of the formation of the 

 International Committee for Investigating the Constituents of Steel.* 

 This committee has apparently ceased to exist ; no authoritative 

 report seems to have been issued. The publication of Breuil's 

 work has been long delayed through the opposition of H. le Chatelier 

 and L. Gnillet, who do not appear to have been satisfied with the 

 methods adopted and the experimental programme followed. The 

 author examined, microscopically and mechanically, six steels (carbon 

 0'38, 0-70, 0-85, 1"20, 1*40, 1'80 p.c), and some cast-irons 

 and cemented steels. Samples (three different sizes) of each were 

 quenched from 650°, 750°, 850°, 1050°, and 1200°-C, and were examined 

 as quenched, and also after re-heating to 225°, 355°, and 455° C. The 

 temperatures were taken by a thermocouple, in conjunction with a 

 Callendar recorder arranged as a potentiometer. Considerable decar- 

 burisation occurred in heating, so that the true structures were only 

 obtained in the central portions of the larger pieces. The author gives 

 numerous details of methods of polishing, etching, and preparation of 

 polishing powders. Powdered talc was employed for polishing, and 

 KourbatofFs reagents were used. The most remarkable conclusions 

 reached by the author relate to the constitution of troostite, which is 

 held to be finely divided graphitic carbon resulting from the decompo- 

 sition of cementite before passing into solution in the iron. Cementite A 

 is the cementite of pearlite, while cementite B is massive. Sorbite is a 

 pearlite of fine emulsified granules of cementite. Martensite is a com- 

 plete but unsaturated solution of cementite A in ferrite. Hardenite is 

 a saturated martensite. Austenite is hardenite, in which is dissolved 

 cementite B. Osmondite is an incipient solution of the granules of 

 sorbite, which are surrounded bv troostite. The changes which occur 

 when an annealed steel is heated are as follows : — Towards 700° C. the 

 sorbite or pearlite granulates and the granules enlarge, then begin to 

 dissolve in the ferrite, apparently with some difficulty, for the larger 

 grains, more slow to dissolve, give off carbon by dissociation. It is 



* Rev. de Metallurgie, iv. (1907) pp. 1027-36 (33 figs.). 



t Bull. Soc. Industrie Minerals, ser. 4, vi. (1907) pp. 553 683 (18 figs, and 333 

 photomicrographs). See also Metallurgie, v. (1908) pp. 59-60,96-9, 105-14(335 figs.). 

 % See this Journal, 1905, p. 534. 



