ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 259 



fracture. The surface furrows (piezoglyps) found on meteorites are 

 ascribed to erosive action of gases on originally rough and irregular 

 fractured surfaces in their passage through the atmosphere. J. E. Stead, 

 and others, contributed to the discussion. 



Case-hardening of Mild Steel.* — C. 0. Bannister and W. J. 

 Lambert have heated mild steel bars in a cementing material at 

 871° C. and at 1)82° C. for varying lengths of time. The structure 

 and hardness were investigated both after slow cooling and after 

 re-heating to 843° C. and quenching in water. At 871° C. the carbon 

 content of the outer layer did not increase beyond ■ '.> p.c, while at 

 982° C. the bars became supersaturated on the outside. 



Case-hardening. f — G. S. Scott, in the course of experiments on the 

 influence of time, temperature, and composition of cementing material, 

 has found that the materials which give the most rapid case-hardening 

 effect either contain nitrogen or have the power of utilising atmo- 

 spheric nitrogen. Gaiillet's mixture (60 p.c. wood charcoal, 40 p.c. 

 BaC0 3 ), is very effective. Samples of mild steel, cemented in a non- 

 nitrogenous material (sugar carbon), were found to absorb less carbon 

 than samples (1) cemented in the same way, but previously heated in 

 an atmosphere of ammonia-gas at 550° C, or (2) cemented in the same 

 material through which passed a stream of ammonia-gas. Heating in 

 ammonia-gas was found to produce twinning ; the author suggests that 

 nitrogen induces the formation of y-iron, and that this is the explana- 

 tion of its effect in accelerating; carburisation. 



*& 



Hardened Steels.J — P- Longmuir examined the microstructure of a 

 large number of commercially hardened tools, carbon ' 5 to 2*0 p.c. 

 The good tools were found to consist of hardenite, alone or with 

 cementite or ferrite, and had a characteristic absence of definite 

 structural pattern. The tools spoilt in hardening frequently showed 

 marked patterns, and martensitic, austenitic, and troostitic appearances 

 were noted. The effect of different heating and quenching temperatures 

 on a 1*15 p.c. carbon steel was determined. Uniformity of structure 

 in tool steel is only obtained by quenching in a certain range of 

 temperature. 



Hardening of Steel.§ — L. Demozay states at some length the 

 conclusions, many of which are of an obvious character, drawn from 

 extensive series of experiments, in which the rates of heating and of 

 cooling of steel, under widely varying conditions, were determined. 

 The heating curves given are of value. The transformation point on 

 heating varies between two temperatures, the maximum value being the 

 transition temperature at the centre of a very small sample rapidly 

 heated, the minimum that of the surface of a large sample slowly 

 heated. For a given temperature of heating-bath the maximum rate of 

 heating diminishes from outside to centre of the sample. 



* Journ. Iron and Steel Inst., lxxv. (1907, 3) pp. 114-19 (22 photomicrographs). 



t Tom. cit., pp. 120-36 (12 figs.). 



X Tom. cit., pp. 137-43 (lb photomicrographs). 



§ Tom. cit., pp. 144-78 (49 figs.). 



