in Iron and Steel during Heatiny and Cooling. 393 



During the heating of a \\iro, wlieii in its commercial state, there 

 are one expansive and two contractive tendencies exerting their 

 powers at one and the same time ; and conversely, in the act of cool- 

 ing, there are one contractive and two expansive inflnences at work, 

 \az. : — ordinary cooling contraction ; an expansion which is the oppo- 

 site of the temporary contraction of heating — the cooling-Icicle ; and 

 an expansion which is the opposite in nature, but not necessarily 

 in amount, of the contraction of heating — crystalline expcuision. 

 In every case the interpretation of the general result depends 

 upon the acciu'ate estimate of the extent to which these inter- 

 acting forces have modified each other's effects. Thus during the 

 heating we have a temporary and a permanent contraction, assisting 

 each other to oppose the dynamical expansion. In cooling, on the 

 other hand, we have two expansions, one of which is temporary and 

 the other permanent, opposing the dynamical contraction of cooling. 



It will be desirable to carefully define the nature of these re- 

 spective influences, and the terms which will be used to distinguish 

 them in this research. 



I. Dynamical Expansion. — This term will be used to distinguish 

 the ordinary dilatation produced in bodies generally, by raising 

 them from a given temperature to a higher one, and which is exactly 

 counterbalanced by contraction when the original temperature is 

 regained. It is proposed to call this kind of contraction "the dy- 

 namical contraction."' 



II. Contraction of Heating. — In the diagrams representing second 

 heatings [vide fig. 2] we get the first inkling of the existence of a 

 contractive or shortening influence excited by heat simultaneously 

 with the expansion. In the diagrams of annealed and hardened 

 \^TLre [vide figik 3, 4, 5] this influence becomes more obvious still. 

 In these cases it is seen to greatly mash 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 

 (w^hich 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 coolins : — 



