the Recalescence of Iron. 115 



the bringing of the liquid water into contact with small crystals 

 of ice which have formed on the side of the vessel. Besides, 

 the internal friction of the iron is very great above the tem- 

 perature of 1000° C, and the vibrations may not have been 

 made sufficiently near the point of recalescence' 55 '. 



Experiment VI. — This experiment was made with a view of 

 ascertaining whether the reglow would occur at a lower tem- 

 perature the greater the amount of retentivity of the specimen. 

 A specimen of well-annealed Swedish iron wire showed no 

 trace of reglow. This may have been for two reasons : — In 

 the first place, if the reglow occurs very near the critical tem- 

 perature the very brightness of the wire renders detection 

 difficult ; in the second, the amount of potential energy sud- 

 denly converted into kinetic energy is less. The author does 

 not believe that in this specimen there was no recalescence, for 

 the following reasons : — Whenever recalescence is very marked 

 the sudden changes which have been mentioned as taking place 

 in a wire cooling whilst under stress and strain are also well 

 marked ; and, conversely, if there is no sudden change in a 

 wire under stress or strain there is no recalescence : the 

 recalescence and the sudden changes seem to be inseparable 

 companions. Now, with the particular specimen of iron in 

 question, the sudden changes above alluded to could be 

 detected ; but they evidently occurred at a much higher tem- 

 perature than with those specimens of iron and steel which 

 showed recalescence plainly. 



As far as could be ascertained from an examination of 

 about a dozen different specimens of iron and steel, those 

 specimens which seemed most capable of being softened by 

 the process of annealing were those in which the phenomenon 

 of recalescence was least marked. When recalescence is 

 manifest, the temperature at which it appears seems to vary 

 considerably with different specimens : with some this tempe- 

 rature would be at least as high as 800° C, with others at 

 least as low as 550° C. 



Views of Professor G. Forbes and Mr. H. F. Newall 

 concerning Recalescence. 

 According to Professor Forbes f, the phenomenon of re- 

 calescence is due to the fact that at a certain temperature 

 there is a sudden increase of the thermal conductivity of iron. 

 As the metal cools from a white heat, the difference between 



* It was impossible to continue the agitations right up to the poin of 

 recalescence, as the motion prevented the observer from seeing whether 

 the reglow took place or not. 



t Proc. R. S. E. April 6, 1874. 

 12 



