ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 437 



equilibrium diagram are traversed, the finally resulting phases may not 

 be in equilibrium. Numerous instances of these various effects are de- 

 scribed and illustrated by photomicrographs. A cast alloy of 05 p.c. 

 copper 5 p.c. tin was annealed at 800° C. The specimen was polished 

 and etched with a mixture of ammonia and hydrogen peroxide after J, 1, 

 2, and 3 hours' annealing. The gradual disappearance of the cored struc- 

 ture was thus followed, the solid solution became homogeneous. The 

 phenomenon of coalescence, due to the action of surface tension, is 

 apparent in the changes occurring when hypereutectoid steels are an- 

 nealed. The formation of twin crystals by deformation followed by 

 annealing is explained upon the assumption of the occurrence of " pro- 

 portional translation," a particular mode of slipping in crystals upon 

 their cleavage planes. 



Influence of Intercrystalline Cohesion upon the Mechanical 

 Properties of Metals.* — J. C. W. Humfrey advances a modification of 

 Rosenhain and Ewen's theory as to the formation and structure of the 

 intercrystalline cement which joins the irregularly outlined crystals of 

 which metals are built up. It is suggested that if the solidification of 

 a metal takes place sufficiently slowly, there is formed between two 

 crystals which have grown towards each other, a layer in which the 

 molecules are so arranged as to pass by gradual small displacements from 

 the orientation of one crystal to that of the other. A similar layer 

 probably exists between crystals which have been formed by recrystalli- 

 zation in the solid state, if equilibrium between the crystals is complete. 

 If, from rapid cooling or other causes, true equilibrium has not been 

 established in solidification or in solid recrystalhzation, the crystals may 

 be separated by an amorphous layer. A study of the behaviour of slip- 

 bands at or near the intercrystalline boundaries has afforded support to 

 the author's views. Pure Swedish iron, polished and lightly etched, was 

 microscopically examined during strain. Slip-bands first appeared in 

 the central parts of the crystals, and only spread towards the boundaries 

 as the straining became severe. Slip-bands approaching a boundary at 

 an angle became narrower and tended to bend so as to approach the 

 boundary at a smaller angle. Occasionally slip-bands were observed to 

 cross a boundary. In such a case the sKp-band persisted unchanged in 

 direction and was apparently little affected by the boundary. Such an 

 observation is explained on the supposition that the two crystals, though 

 not identical in orientation, have one common ghding plane. 



Etching at High Temperatures.-f — H. Hanemann holds that 

 the method by which a section, polished at ordinary temperatures, is 

 heated, and etched at a high temperature by the application of a gaseous 

 or liquid reagent, does not give a true picture of the structure at the 

 temperature in question. If re-crystallization takes place during heating, 

 the polished surface, which was made up of sections through crystals at 

 the ordinary temperature, consists of crystal surfaces at the high tem- 

 perature. The phenomena of segregation and of formation of new 



* Iron and Steel Inst. Carnegie Scholarship Memoirs, v. (1913) pp. 86-99 

 (8 figs.). 



t Int. Zeitschr. Metallographie, iii. (1912) pp. 176-8. 



Aug. Wth, 1913 2 G 



