668 SUMMARY OF CURRENT RESEARCHES RELATING TO 



differences of temperature (clue to the difference in direction of the 

 main current) between corresponding points in the two bars, were 

 measured by means of thermo-electric couples of copper and German- 

 silver, connected to a sensitive galvanometer. Similar measurements 

 were taken with the main current reversed. Numerous details of the 

 method and the precautions adopted are given by the authors. The 

 following values were obtained for the Thomson effect coefficient v, 



| - 757 X 10- 10 mean, from 13° to 90° 



approximately < — 715 „ „ „ 13° „ 51° 



( ~ 793 „ „ „ 51° „ 90* 



Influence of Nitrogen on Iron and Steel.* — H. Braune points out 

 that the abnormal brittleness frequently [met with in iron and steel, 

 especially in basic products, cannot be ascribed to the presence of large 

 percentages of phosphorus, sulphur, or other elements, the effect of which 

 is well known. As the result of six years work, the author states that 

 this brittleness is caused by the presence of nitrogen absorbed by the 

 metal in the processes of manufacture. The nitrogen exists as nitride 

 of iron in solid solution in ferrite. To determine the effect of nitrogen 

 on the quality of the metal, the author heated bars of high-grade iron 

 and steel in ammonia gas at 800° C. for varying periods. Different 

 proportions of nitrogen were absorbed ; the bars were then annealed to 

 bring about a regular diffusion of nitrogen through the metal. Curves 

 are given showing the effect of increasing proportions of nitrogen on 

 the mechanical properties of (1) soft iron containing ' 06 p.c. carbon, 

 (2) steel containing 1 " 15 p.c. carbon. The effect is wholly injurious : 

 0*08 p.c, or more, nitrogen renders soft iron brittle ; in the case of the 

 high carbon steel the same effect results with only 0*03 p.c. nitrogen. 

 Photomicrographs are given showing the remarkable changes in struc- 

 ture brought about by increasing percentages of nitrogen. 



In an editorial note appended to the above article, H. le Chateliert 

 cites a number of facts — such as the well known difference in properties 

 between acid and basic steel of the same composition — as to hitherto 

 unexplained departures from established laws governing the relation 

 between physical properties, chemical composition, and thermal treat- 

 ment of steel. These irregularities have been attributed to the presence 

 of elements, such as hydrogen and oxygen, which are difficult to esti- 

 mate. H. Braune's researches suggest that the explanation may be 

 sought in the presence of nitrogen. H. le Chatelier gives some results he 

 has obtained in endeavouring to establish a relation between brittleness 

 of metals and their microstructure. He suggests that nitrogen, when 

 present in moderate proportions, may have the effect of facilitating the 

 development of brittleness under unsuitable thermal treatment, rather 

 than that of rendering the metal hopelessly bad. 



The method of estimating nitrogen in iron, together with further 

 details, is given elsewhere by H. Braune.J 



* Rev. Metallurgie, ii. (1905) pp. 497-502 (7 figs.), 

 t Tom. cit., pp. 503-7 (3 figs.). 



t Op. cit., Extraits, ii. (1905) pp. 361-4 (4 ;figs.). See also Braune, H., "Sur 

 le role de l'azote dans les fers et aciere." Bale. Editeurs, Walz et Mieville, 1905. 



