390 



Royal Society : — Prof. Norrls on Molecular Cltanges 



it became necessary, -with a view to learn how far it bore upon the 

 subject in hand, to inquire into the nature of a pecuh"ar fact which 

 was observed by his friend Mr. Gore, and published in the Pro- 

 ceedings of theEoyal Society, Jainiary 28th, 1869, under the title 

 of "A Momentary Molecular Chanoje in Iron Wire." In this 

 communication it was shown that if a strained iron wire were 

 heated to redness by a voltaic current or other means, on being 

 allowed to cool, the contraction of cooling was at a certain point, 

 and for a limited period, arrested by an action of elongation. 



"AVith wires of iron 0-65 mil'lim. thick (No. 23) and 21-5 

 centims. long, strained to the extent of ten ounces or more, and 

 heated to full redness, the phenomenon was clearly developed. Por 

 example, the needle of the instrument went with regularity to 18-5 

 of index-plate ; the current was then stopped ; the needle instantly 

 retreated to 17'75, then as quickly advanced to 19'75, and then 

 went slowly and regularly back, but not to zero. After shutting 

 off the source of heat, the wire contracted 0*75 millim., and then 

 expanded 2 millims., so that it. returned 1-25 millim. 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 duriug the occurrence by the strain, was 

 therefore 2 millims. If the temperature of the wire Mere not suffi- 

 ciently high, or the strain upon the wire not enough, the needle 

 went directly back without exhibiting the momentary forward move- 

 ment. The temperature and strain required to be suflicient to 

 actually stretch the wire somewhat at the higher temperature. A 

 higher temperature with a less degree of strain, or a greater de- 

 gree of strain \^ith a somewhat lox^er temperature, did not develop 

 the phenomenon. The wire was found to be permanently elongated 

 on cooling." 



Mr. Gore further observes : — "The molecular change evidently 

 includes a diminution of cohesion at a particular temperature 

 daring 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 suflicient to pro- 

 duce it ; the condition of cooling must also be included. The phe- 

 nomena 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 retrogression 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 tem- 

 perature at which the elongation takes place on cooling. Eeleasing 

 the tension of the spring, the forward motion on cooling is, as 



