BURGESS AND CROWE: NOTE ON RANGES OF PURE IRON 329 



METALLOGRAPHY. — Preliminary note on the critical ranges, 

 A3 and A2, of pure iron. G. K. Burgess and J. J. Crowe, 

 Bureau of Standards. 



In view of the recent publication by Professor Carpenter of 

 his very interesting results^ on this subject and considering its 

 theoretical importance, we deem it advisable at this time to give 

 a brief account of some preliminary observations taken in 1911 

 and 1912 on the location of the critical ranges in pure iron both 

 on heating and cooling. We had intended deferring publication 

 until results were ready on a specially pure sample of iron pre- 

 pared here by Mr. J. R. Cain. Since, however, with five samples 

 of iron of the same order of purity as the single one used by 

 Professor Carpenter (99.97 per cent Fe) we are not able to reach 

 the same conclusion he does, namely, ''that were it possible to 

 remove the last traces of impurity, iron would cease to show even 

 these faint indications of Ar2," it may be of interest to put on 

 record now some of the results so far obtained. Cooling and 

 heating curves of five samples of electrolytic iron have been taken 

 in vacuo, three samples being separate preparations in the form 

 of cathode plates from Prof. C. F. Burgess of Wisconsin; and 

 two samples remelted by us in vacuo to remove gases, one pre- 

 pared by the Langheim-Pfanhauser A. G. furnished by Dr. H. 

 Goldschmidt and one a sample of the C. F. Burgess iron. The 

 samples weighed from 21 to 31 grams. 



Two methods of locating the critical ranges were used simul- 

 taneously, requiring two observers. The first was the Osmond 

 inverse-rate method, times being recorded to 0.1 second on a 

 cylindrical chronograph and temperatures taken in steps of 2°C. 

 by means of a Diesselhorst potentiometer. 



The second was the Roberts-Austen differential method with a 

 platinum neutral and reduced by Rosenhain's derived differential 

 method. The plotted curves should be strictly similar in shape, 

 and in practice, unless something is the matter, they are similar. 



Using a specially designed liquid rheostat, it was possible to 



^ H. C. H. Carpenter, The critical ranges of pi^re iron. Iron and Steel Insti- 

 tute, May, 1913. 



