84 SUMMARY OF CURRENT RESEARCHES RELATING TO 



of any desired size, up to the whole area of the specimen, which then 

 consisted of one crystal. Striking such a plate of tin with the point of 

 a needle produced : (1) on the face opposite to the face penetrated, a 

 cross in relief, with broad arms ; (2) two or three series of bright bands, 

 parallel, of width up to 0*5 mm., and reaching from the impression 

 made by the needle, to the boundary of the crystal. These twinned 

 lamellae passed right through the crystal. Their faces, originally in the 

 same plane as the general surface of the plate, now formed an angle of 

 several degrees with it. Further phenomena observed are described. 

 The " cry " of tin appears to be caused by the formation of twinned 

 crystals. 



Dilute Solutions of Aluminium in Gold.* — C. T. Heycock and 

 F. H. Neville have determined the equilibrium diagram of the aluminium- 

 gold system for the range to 5 p.c. aluminium, and describe the 

 microstrncture of the numerous alloys examined. From to 2 p.c. 

 aluminium the a solid solution only was found. From 2 to 3 p.c. the 

 alloys consisted of a and ft, ft being stable only above 424° C. A sub- 

 stance D may be the compound Al 3 Au 8 . Polished and etched surfaces 

 of ft that have been chilled at a high temperature show, under high 

 magnification, groups of fine parallel lines, the direction of the lines 

 changing from grain to grain so as to give the effect of shading. This 

 is due to a laminated structure in the ft, perhaps to an incipient decompo- 

 sition. When ft is slowly cooled it breaks up at 424° C. (the eutectoid 

 temperature) into a complex of a and of D. The etching reagents 

 used were bromine water and aqua regia ; the two gave practically the 

 same pattern. 



Nitrogen in Steel. f — A peculiar structure found in the welded 

 portions of electrically-welded iron plates led B. Strauss to undertake an 

 investigation upon nitrogen in steel. These welded portions contained 

 up to 0*12 p.c. nitrogen, while plates welded by means of acetylene 

 contained 0"02 p.c. nitrogen. The nitrification of iron specimens in a 

 current of ammonia gas begins below 300° C, and proceeds most rapidly 

 between 600° and 800° C. In this way are obtained layers differing in 

 nitrogen content and in structure. The outermost layer of a nitrified 

 specimen of pure iron consisted of the nitride Fe 4 N 2 . Below this was 

 a layer, having a pearlite-like structure, in which both carbon and 

 nitrogen were present, the carbon being obtained from pyridine, an 

 impurity in the ammonia used. The next layer had an acicular structure, 

 which was also present in the electrically-welded plates. The needles, 

 formerly regarded as consisting of an iron-nitride, were found to be 

 twinned lamella? in the nitrogen-containing ferrite crystals. When 

 carbon steels were nitrified, another constituent, appearing as light 

 brown specks in the etched specimens, was formed. The iron nitride 

 was readily decomposed by heating : when elements such as silicon. 



* Phil. Trans., Series A, ccxiv. (1914) pp. 267-76 (26 figs.), 

 t Stahl und Eisen, xxxiv. (1914) pp. 1055-6. 



