On the Recalescence of Iron. 103 



value (28) gives 



Since now the magnitude F can no more contain the vari- 

 ables £, rj, £, the term multiplied by £, that multiplied by 77, 

 and that multiplied by £ in the last equation must separately 

 vanish, whence at once the well-known hydrostatic differential 

 equations follow. Consequently this line of argument, which 

 Prof. Tait designates a " remarkable proceeding," does not in 

 any way involve any special assumption, but is a logical treat- 

 ment of the general equations of the kinetic theory of gases ; 

 and I am confident that those of my. readers who honour the 

 foregoing with sufficient attention will not be able to raise any 

 objection to it. (Cf. on this point also Maxwell, 'Nature,' 

 vii. pp. 525 & 535, October 23, 1873.) 



XII. The Recalescence of Iron. 

 By Herbert Tomlinson, B.A.* 



Introduction. 



MANY specimens of iron and steel, when they have been 

 raised to a white heat and are cooling, exhibit at a 

 certain stage a remarkable phenomenon — the metal, to all 

 appearances, receives a sudden accession of heat, and reglows. 

 This phenomenon was discovered by Professor Barrett, and is 

 frequently designated the " recalescence " of iron. The author 

 believes the recalescence of iron to be due to a sudden phy- 

 sical change, the event of which has been retarded by what 

 he will call " subpermanent retentivity." The retentivity 

 of a substance may be defined to be that property by virtue 

 of which the substance does not immediately recover from 

 the strain produced by a stress when the stress has been 

 removed. If the original strain produced by the stress be 

 large, the residual strain will consist of two parts: one per- 

 manent, which will not disappear even after the lapse of any 

 length of time ; the other subpermanent, which will disap- 

 pear after a greater or less interval of time, and which may 

 be aided in doing so by molecular vibrations set up by 

 mechanical or any physical agency. Subpermanent reten- 

 tivity appears with a great variety of strains : — when a wire is 

 twisted or bent, when a piece of iron or steel is magnetized, 

 when a Leyden jar is charged, and when glass has been raised 

 in temperature. The last is an example of subpermanent 



* Communicated by the Physical Society : read December 10, 1887. 



