Temper' dure of Iron and Steel during Recalescence. 179 



firmed by telescopic readings for iron containing 0'3 per cent, 

 to 0*1 per cent. 



The expansion at D' seems, in iron with a small percentage 

 of carbon, to consist of two separate moments, D' 2 and ~D\. 

 In soft steel they partly synchronize and produce the jerk at 

 D' characteristic of this kind of steel. In hard steel they 

 have completely synchronized*. 



The relation of the critical points D and D ; to each other. — 

 The three curves in fig. 3 represent changes in length of one 

 and the same rod of Bofors wire containing 0'6 per cent, of 

 carbon which was heated to different degrees of temperature 

 and between each heating was allowed to cool slowly. The 

 lower one of these curves represents the longitudinal changes 

 of the rod when the flame was extinguished immediately 

 before, the middle one at the beginning, and the upper at the 

 end, of the anomalous contraction at D. 



These curves, as well as other observations not given here, 

 show : — 



(v.) The expansion at D' does not appear during the cooling 

 process unless the contraction at D during the heating pro- 

 cess has partially or completely taken place. The expansion 

 at D' is less when the rod has only been heated to a degree 

 of temperature corresponding to the contraction at D than 

 when the rod is heated to a higher degree of temperature. 



The effect of long heating upon the critical points 1) and D'. 

 — The lower curves in figs. 4 and ft represent longitudinal 

 changes in rods of Bofors wire containing O'l per cent, and 

 0'6 per cent, of carbon which were heated for six hours in an 

 assay-furnace up to temperatures above the fusing-point of 

 gold, and which thereafter were allowed to cool very slowly. 

 The upper curves in the same figure represent changes of 

 length in rods containing the same percentage of carbon, but 

 which have not undergone such long heating. 



These curves show : — 



(vi.) Protracted heating to a high degree of temperature, 

 followed by slow cooling, diminishes in a large degree the 

 magnitude of contraction at D and the expansion at 1)'. The 

 contraction at D lasts longer after the prolonged heating, and 

 does not finish in iron containing 0*1 per cent, of carbon 

 before the flame is extinguished. 



Observations made by the aid. of telescope and scale show 

 that the contraction at D and the expansion at D' decrease 



* First observed by Prof. Barrett in 1875 " for some specimens of steel 

 wire," and mentioned in ' Report (1890) on Molecular Phenomena in 

 Magnetized Iron,' p. 2. 



