ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 589 



of a layer upon the surface of the primary crystal of that constituent 

 present as primary, the other constituent solidifying as an envelope 

 surrounding the primary. 



Hardening of Metals.* — C. A. Edwards and H. C. H. Carpenter 

 develop a theory of the cause of the hardening of steel and other alloys 

 upon quenching. In the operation of quenching such alloys, severe 

 stresses are set up which cause pronounced crystal twinning. This 

 twinning appears to be directly connected with the intensity of the 

 thermal changes, such as the Ar x inversion, which occur when the mass 

 is slowly cooled, but are suppressed by quenching. At all the surfaces 

 of slip upon which twinning occurs, amorphous layers are formed, 

 similar to those developed when a metal is hardened by cold-working. 

 The hardening resulting from quenching is due to the presence of the 

 amorphous layers. The authors regard martensite as austenite in which 

 twinning has occurred. 



Theory of Hardening.! — A. M'Cance gives the results of experi- 

 mental work on which he bases a theory of the hardening of steel 

 by quenching. /3-iron is not a separate allotropic condition, but 

 is a-iron which has lost its ferromagnetism through purely thermal 

 causes. On cooling, the change from y to a-iron takes place with very 

 high velocity, and cannot be appreciably retarded by quenching. A 

 quenched steel contains carbon in the state of solution, which retains a 

 portion of the iron in the y condition. Most of the iron is in the a 

 condition, but is hard owing to its state of interstrain, resembling the 

 state of a metal hardened by cold-working. The author describes a 

 peculiar structure developed in a disk of steel, containing 0*86 p.c. 

 carbon, clamped between two larger masses, heated, and quenched, so 

 that the disk was cooled only from its edge. A number of concentric 

 zones, alternately hard and soft, and appearing alternately light and 

 dark in the polished and etched surface, were found surroundine; the 

 dark-etching centre of the disk. 



Cast-iron of Unusual Structure. $ — K. W. Zimmerschied describes 

 the structure of a cast-iron consisting wholly of pearlite and graphite, 

 and compares it with cast-irons having more usual structures. 



Titanium nitride in Steel.§ — G. F. Comstock has investigated the 

 constitution of certain hard non-metallic inclusions found in rails made 

 from steel, to which ferrotitanium had been added. They were never 

 found in rail steel which had not been treated with titanium. The in- 

 clusions were minute, were not segregated in groups but always uniformly 

 distributed through the section of the rail, and were not elongated in the 

 direction of rolling. They were usually rounded in shape, and sometimes 



* Journ. Iron and Steel Inst., lxxxix. (1914, 1) pp. 138-91 (28 figs.). 

 t Journ. Iron and Steel Inst., lxxxix. (1914, 1) pp. 192-265 (24 figs.). 

 j Foundry, xliii. (1914) pp. 404-8 (11 figs.). 

 § Met. and Chem. Eng., xii. (1914) pp. 577-80 (16 figs.). 



