132 M. G. E. Svedelius on the Changes in the Length and 



40 times, a couple of minutes each time, being allowed to 

 cool slowly after each heating, only amounted to | of its 

 magnitude the first time the rod was heated. In a rod of 

 electrolytic iron the magnitude of the expansion at D' de- 

 creased very rapidly with every renewed 

 heating ; and after the 50th heating no ^ 

 trace either of the critical point D or D' ~~, / 

 could be discovered*. The iron was now if 

 brittle, with coarse crystalline, glossy ta - 

 fracture, and showed all the qualities 

 characterizing burnt iron. Neither in 

 any other burnt iron examined by me 

 could I discover any critical points. 



Anomalous changes of length in re- 

 tempering hardened Steel. — The upper 

 curves in figs. 6-11 represent changes 

 of length in rods hardened in cold water 

 after being heated to a bright red heat ; 

 the lower curves represent changes in 

 length in the same rods after being 

 allowed to cool slowly after the previous 

 heating. 



These, and other curves not reproduced 

 here, show : — 



(vii.) The expansion to the critical 

 point D does not take place as regularly 

 in the hardened as in the annealed rods. 

 The longitudinal curve of the hardened 

 rod shows irregular jerks which appear 

 after a few seconds' heating. In rods con- 

 taining 0'9 per cent, to 0*7 per cent, of 

 carbon there are two similar jerks or 

 critical re-tempering points, di and d 2 ; in 

 rods with a lower percentage of carbon, 

 one critical re-tempering point d, which 

 appears clearly when the percentage of 

 carbon runs up to 0*6 percent, to 0*4 per 

 cent., and weaker, if it does not disappear 

 entirely, when the percentage of carbon 

 is lower. The two re-tempering-points d 1 

 and d 2 seem to correspond to each other, 

 (viii.) The contraction at D begins earlier 

 in the hardened than in the annealed rod. 



* This confirms the statement long ago made by Prof. Barrett that 

 in very pure iron the anomalous contraction and expansion could be 

 "washed out ' as it were by repeated heating and cooling. 



