ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 671 



and the phase doctrine. The important part played by the Microscope 

 in its development should not, however, be forgotten. While the phase- 

 rule furnishes information regarding stable equilibria, the Microscope is 

 almost the only means of investigating metastable conditions of alloys. 

 Starting with a diagram of the critical points of iron-carbon alloys, 

 agreeing closely with that given by Roberts- Austen, the author describes 

 the changes which take place when cooling is sufficiently slow to permit 

 the attainment of stable equilibrium. When the rate of cooling is 

 somewhat accelerated, stable equilibrium does not result. Assuming 

 that by rapid quenching from a temperature T the alloy is retained, at 

 a lower temperature t, in a condition corresponding to stable equilibrium 

 at T, a number of cases are taken and the final constitution of the alloy 

 inferred. Such complete supercooling, however, is not possible in the 

 case of iron-carbon alloys. The condition of an alloy rapidly cooled 

 from a temperature T to t is unstable, and is intermediate between the 

 condition stable at T and that stable at t. T is assumed to be above, 

 and t below the critical range. Transition constituents, which must not 

 be considered as phases, are thus formed. Martensite and troostite are 

 well known examples of such constituents. Possibly austenite may also 

 belong to the same category, instead of being, as Osmond regards it, a 

 separate phase. As a means of distinguishing troostite from martensite 

 and other constituents microscopically, 1 p.c. hydrochloric acid in absolute 

 alcohol is recommended as an etching reagent. The author gives his 

 reasons for doubting the occurrence of the transformation — 



martensite + graphite ^ carbide, 



which, according to Roberts-Austen and Roozeboom, takes place at 

 1000° C. Their view is not supported by experimental data. An 

 alternative theory is advanced, the condition corresponding to the two 

 phases, iron and graphite being acoeptecl as stable, while the existence of 

 carbide is due to supercooling. Carbide (cementite) is thus a metastable 

 form. 



Metallography applied to Foundry Work.* — A. Sauveur describes 

 the various methods suitable for differentiating the constituents in a 

 microscopical section of cast-iron, 10 p.c. nitric acid in absolute alcohol 

 is recommended as an etching reagent. Graphite may be distinguished 

 by examination of the section after simple polishing. 



On the Magnetisation and the Magnetic Change of Length in 

 Ferromagnetic Metals and Alloys at Temperatures ranging from 

 -186° C. to +1200° C-t — K. Honda and S. Shimizu have measured 

 the magnetisation and magnetic change of length of pure iron, nickel, 

 cobalt, tungsten steel, and 12 specimens of nickel steel containing 

 from 24 p.c. to 70 p.c. nickel, at the temperature of liquid air, at 

 1200° C, and at intermediate temperatures. Temperatures between 

 — 186° C. and —15° C. were obtained by surrounding the specimen by 

 a jacket containing liquid air. Uniform slow cooling thus resulted. 

 High temperatures were obtained by inserting the specimen in a platinum- 

 wound electric resistance tube furnace. A platinum German-silver 



* Iron and Steel Mag., x. (1905) pp. 29-32 (2 figs.). 

 t Journ. Coll. Sci. Tokyo, xx. Art. 6, pp. 1-63 (4 pis.). 



