128 



Prof. Norris on Molecular Changes in Iron [Apr. 12, 



beyond the original point of heating, and then resumed its course towards 

 zero, which it failed to reach. The length of the kick, plus the amount 

 of stretch, occasioned during the occurrence by the strain was therefore 

 2 millims. If the temperature of the wire was not sufficiently high, or 

 the strain upon the wire not enough, the needle went directly back with- 

 out exhibiting the momentary forward movement. The temperature and 

 strain required to be sufficient to actually stretch the wire somewhat at 

 the higher temperature. A higher temperature with a less degree of 

 strain, or a greater degree of strain with a somewhat lower temperature, 

 did not develop the phenomenon. The wire was found to be permanently 

 elongated on cooling." 



Mr. G-ore further observes : — " The molecular change evidently includes 

 a diminution of cohesion at a particular temperature during the process 

 of cooling ; and it is interesting to notice that at the same temperature 

 daring the heating-process no such loss of cohesion (nor any increase of 

 cohesion) takes place ; a certain temperature and strain are therefore 

 not alone sufficient to produce it ; the condition of cooling must also be 

 included. The phenomena which occur during cooling are not the exact 

 converse of those which take place during heating." 



The subject was subsequently taken up by Professor Barrett, of Dublin, 

 who, in a paper to the ' Philosophical Magazine ' in 1873 (vol. xlvi. p. 472), 

 showed, by the use of the mirror, that during the heating of the wire a 

 converse action to that which takes place during cooling occurs. He 

 says : — " During the heating of the wire a slight and momentary retrogres- 

 sion of the beam was noticed at the temperature corresponding to the 

 powerful jerk that occurred on cooling. 



" The temperature at which the momentary jerk occurs seems to be 

 lower in thick wires than in thin ones. The momentary retraction, as 

 closely as can be judged, takes place at the same temperature at which the 

 elongation takes place on cooling. Releasing the tension of the spring, 

 the forward motion on cooling is, as might be expected, much lessened, 

 whilst the jerk back is scarcely affected. Increasing the tension of the 

 spring, the forward jerk is correspondingly increased, and the backward 

 jerk diminishes and can be made to disappear. Without the spring, an 

 iron wire can be seen by the naked eye to undergo a momentary con- 

 traction during heating, and a momentary and more palpable elongation 

 during cooling. 



" All kinds of iron do not exhibit this behaviour ; and some show it in 

 a more or less marked degree. I have not been able to detect any change 

 in certain specimens of good soft iron wire ; but in hard iron wire, and 

 notably in steel wire, it is very apparent. 



"The wire, moreover, requires to be raised to a very high temperature 

 before the jerk is seen on cooling. I have not observed the momentary 

 elongation on cooling when the wire has only been heated to a point just 

 beyond that at which it would otherwise occur. 



