ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 351 



scale and metal after polishing only, and after etching with the cupric 

 re-agent, show that oxidation follows the boundaries of the crystal 

 grains, and that the residual metal in these grains is enriched in nickel. 



Effect of Cuprous Oxide on the Development of Re-crystallized 

 Grain by Annealing Cold-worked Copper.* — C. H. Mathewson and 

 G. V. Caesar have investigated this subject by annealing for one hour 

 »t temperatures of 700°, 775°, 850°, and 925°, a series of copper-oxygen 

 alloys, ranging from pure oxygen-free copper to a hypereutectic alloy 

 containing Q-iA p.c. oxygen, all of which had been previously cold- 

 worked by rolling. Photomicrographs were then prepared of each 

 sample after cold-working and after each annealing. These show that 

 the growth of the re-crystallized grains on annealing is progressively 

 retarded as the oxygen content increases, e.g. the number of grains per 

 unit of surface after annealing for one hour at 925° was four times as 

 great in pure copper as in an alloy containing as little as 0*05 p.c. 

 oxygen. The effect is considered to be due to the cuprous oxide 

 imposing a mechanical obstruction to the coalescence of the grains. In 

 alloys of higher oxygen content the grain-growth was noted to be 

 particularly retarded in the eutectic areas. The pure copper was 

 etched with strong ammonia containing a little hydrogen peroxide. For 

 etching the copper-oxygen alloys, the best results were obtained by 

 alternate application of ammonia and hydrogen peroxide and dilute acid 

 ferric chloride. Only slight differences in the hardness of annealed 

 specimens of the same composition were found, despite the large 

 differences in grain-size. 



to' 



Heat Treatment of Large Steel Forgings.t — The difficulties at- 

 tendant upon the heat treatment of large masses of steel as compared 

 with small ones are described by Sir W. Beardmore. These include the 

 variations in the rates of heating and cooling which operate throughout 

 large masses, and the distortion caused on quenching by the very con- 

 siderable internal stresses set up. Structural differences, with corres- 

 ponding differences in mechanical properties, arise owing to the differencs 

 in rate of cooling from the outside to the centre of large forgings. 

 Hastening the cooling — e.g. by oil-quenching — tends to greater uniformity 

 of cooling rate, but this also fails when a certain limiting size is exceeded. 

 A comparison is given of the microstructure and mechanical properties 

 of the outside and centre of an oil-hardened steel shaft 18 in. in 

 diameter in illustration. In heating large forgings of varying cross- 

 section the parts of smaller cross-section, which attain the furnace 

 temperature long before the parts of larger cross-section, may become 

 coarse in grain-size and consequently weak and brittle, unless special 

 precautions are taken to enable the whole forging to attain the desired 

 temperature simultaneously. 



* Inst. Zeitschr. Metallographie, ix. (1916) pp. 1-20 (24 figs.), 

 t Journ. Inst. Mech. Eng., 1917, pp. 215-24 (4 figs.). 



