Stress or Permanent Strain to a Bright-red Heat. 265 



heat. Illuminating the index and scale of the apparatus, 

 which was watched by an assistant, it was at once found that 

 the reheating of the wire occurred simultaneously ivith the mo- 

 mentary elongation." Professor Barrett also ascertained that, 

 in order to bring out the phenomenon, it was necessary pre- 

 viously to heat the wire to a white heat ; and, further, that 

 wherever the momentary expansion of the wire is feeble or absent, 

 there likewise this recalescence is also feeble or absent. 



In my own mind I have little doubt that this beautiful 

 phenomenon may be accounted for in the following manner: — 

 When the iron has been heated above a bright red and is then 

 cooling, it reaches the critical temperature at which there is a 

 tendency for the sudden change to take place in the perma- 

 nently strained or temporarily stressed wire : the change does 

 not, however, actually take place at this temperature in con- 

 sequence of the so-called coercitive force of the iron. When 

 the iron is very soft and well annealed, the temperature at 

 which the jerk takes place is comparatively near the tempera- 

 ture at which the jerk occurred on heating. But when the 

 iron is hard-drawn or only imperfectly annealed, there may be 

 a considerable difference in the two temperatures. Suppose, 

 then, that the iron has cooled to the temperature at which its 

 magnetic properties are suddenly restored. At this tempera- 

 ture a commotion more or less profound takes place among 

 the molecules ; and this is sufficient to give them a start 

 towards those positions which they have all along, after the 

 higher critical temperature was passed, been trying to assume. 

 When once started the molecular motion continues, the energy 

 of position is rapidly converted into the energy of motion, 

 and this again into the energy of heat, so that the wire once 

 more glows, It by no means follows that the phenomenon 

 of recalescence occurs with every specimen of iron at the 

 critical temperature at which iron loses its magnetic proper- 

 ties ; indeed I have noticed sometimes more than one evident 

 sudden accession of heat* during the same cooling. More- 

 over, in the case of the very soft iron wire, with which the 

 sudden jerk occurred at a much higher temperature on cool- 

 ing than with the harder specimens, there was no perceptible 

 trace of the phenomenon at a dull red heat. Probably there 

 was a sudden check of the rate of cooling when the jerk oc- 

 curred, though regiowing at this point was not noticed. I 

 did not, however, pay much attention to the matter. 



The view just advanced respecting the phenomenon of 

 recalescence is, I think, justified by the following considera- 

 tions : — 



* Sometimes there is merely a sudden check in the rate of cooling 

 without any sensible recalescence. 



