ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 783 



differences in level, which can be measured. Applying this method to 

 alloys produced by heating iron in boiling sulphur, the author obtains 

 further evidence that FeS is first formed, then FeS 2 . Sulphur appears to 

 form solid solutions with both compounds FeS and FeS 2 . 



Troostite. — H. le Chatelier* remarks that in his article on the 

 constituents of steel f troostite was purposely described vaguely as 

 constituent X in order to avoid controversial matter. The author 

 agrees with Charpy, Grenet, and Benedicks in regarding troostite as 

 pearlite of extremely fine structure. But this has not yet been proved, 

 and is only the most probable hypothesis. The fineness of structure, 

 introducing effects due to surface tension, is the cause of the difference 

 in properties between troostite and pearlite. The thickness of the 

 cementite lamellae in pearlite is of the order of 0' 01^, while the 

 dimensions of the cementite particles in troostite probably do not 

 exceed 0" 00 1/x. The description of troostite as a colloidal solution is 

 unsatisfactory. The term is applied to widely differing mixtures which 

 have the common characteristic of not separating under the action of 

 gravity, while they lack the properties of true solutions. It is difficult 

 to see how a solid body, such as steel, can be correctly described as a 

 colloidal solution. 



Corrosion Tests of Iron and Steel4 — C. Fremont describes the 

 methods of etching for developing the macrostructure of iron and steel, 

 and gives numerous examples of their application. He employs pure 

 hydrochloric acid for rapid etching and dilute sulphuric acid for slow 

 etching. For rendering visible effects due to piping and segregation, the 

 author prefers iodine solution. Examination of macrostructure should 

 be supplemented by shock tests on small notched bars taken from 

 segregated parts. The employment of segregated steel, which has 

 caused many serious accidents through fracture, might be avoided by 

 submitting the metal before use to testing by corrosion. 



Metallography of Quenched Steels.§ — Kourbatoff has studied the 

 transformations of austenite at temperatures up to 445° C. He did not 

 succeed in obtaining pure austenite, but austenitic steels were produced 

 by rapid quenching from high temperatures of samples containing 1 "1, 

 1*6, and 1*9 p.c. carbon. Austenite appears to contain about 2 p.c. 

 carbon. The samples used in the tempering experiments were small 

 bars, one end of which had been heated to fusion in the oxyhydrogen 

 flame, and quenched. Treated in this way, each piece contained several 

 constituents. No change resulted at temperatures below 100° C, even 

 when the heating was continued for two or three months. At 137° C„ 

 a change of structure quickly occurs. At 218° C. austenite is com- 

 pletely transformed in 12 to 18 hours, and at 248° C. in a few minutes. 

 Austenite appears to change directly into troostite, not passing through 

 the intermediate stage, martensite. The author's reagents A and C 

 were used for etching. 



'£>• 



* Rev. Metallurgie, v. (1908) p. 639. t See this Journal, 190S, p. 523. 



% Rev. Metallurgie, v. (1908) pp. 049-703 (41 figs.). 

 § Tom. cit., pp. 704-10 (13 figs.). 



