132 



On Molecular Changes in Iron and Steel. 



[Apr. 12, 



expansion. In the diagrams of annealed and hardened wire [vide figs. 

 3, 4, 5] this influence becomes more obvious still. In these cases it is 

 seen to greatly mask the dynamical expansion. This kind of contraction 

 may be also displayed by protracted heating of the wire, when it exhibits 

 itself at the end of the expansion as a slow, continuous contraction of 

 considerable extent [vide fig. 5]. 



III. Temporary Contraction. — This term is applied to the cooling 

 influence which is excited in the wire during heating, and which, at a 

 certain point, balances the expansion ; for heat being rendered latent at 

 the same rate as it is being absorbed, produces a stop (which indicates 

 that the expansion is arrested and balanced) or a kick (which implies an 

 absolute cooling of the wire from excess of latency). The diminution of 

 the rate of expansion, or the actual shortening produced by this kind of 

 contraction, is of a temporary character, and must not be confounded with 

 the heat-contraction which it accompanies. 



These, then, are the phenomena of heating 



(1) Dynamical expansion ; 



(2) Permanent or heat contraction ; 



(3) Temporary contraction (heating-kicks) ; 



(4) Stretching ; and 



(5) The influence of oxide. 



We have now to consider what influences are present during the 

 cooling : — 



I. Dynamical Contraction. — The opposite of dynamical expansion. 

 II. Permanent Expansion of cooling. — (Crystalline expansion.) 

 III. Temporary Expansion.— The thermal expansion due to the reglow- 

 ing of the wire during cooling. This is the proximate cause of the stops 

 and kicks which occur in cooling. It is evanescent as an opposing force, 

 and must not be confounded with the permanent or crystalline expansion 

 of cooling, of which it is simply the effect, 

 We have, then, in cooling : — 



(1) Dynamical contraction ; 



(2) Permanent expansion of cooling ; 



(3) Temporary expansion (cooling-kicks); and to these may be 



added 



(4) Influence of oxide. 



An exhaustive study of these various conditions has led to the elucida- 

 tion of the nature of hardening, softening, tempering, annealing, &c. 

 of iron and steel, and has further shown that numerical values may be 

 assigned to these states. 



In brief, the research establishes : — 



1. The existence in steel, and in iron containing free carbon, of a con- 

 traction or shortening which is excited by heat, and which proceeds simul- 



