Changes occurring in Iron Wire at a low red Heat. 477 



merit, nor in exhibiting its action to a large class. Wherever 

 the momentary expansion of the wire is feeble or absent, there 



Biff. 2. 



likewise this recalescence, as it might perhaps be termed, is 

 also feeble or absent. I am anxious to procure wires of nickel 

 and cobalt ; for, from the reasons set forth in another paper, the 

 action might certainly be expected to occur in those metals as 

 well as in iron, probably with nickel at a lower and with cobalt 

 at a higher temperature than in the case of iron. 



Besides the molecular changes here detailed, there are indica- 

 tions of the existence of other disturbances during the heating 

 and cooling of an iron wire, notably the emission of a peculiar dry 

 crackling sound, like the crepitation that occurs on magnetizing 

 and demagnetizing iron ; and this, too, occurs at the critical 

 temperature. But so many matters of interest have arisen du- 

 ring this investigation, that the present paper can only be regarded 

 as the results of a preliminary inquiry. 



VI. 



The molecular disturbances to which iron is thus seen to be 

 subject, at a particular temperature, are no doubt associated with 

 an even wider range of phenomena than are here indicated. 

 Professor Tait's experiments, which were read by me after most 

 of the foregoing facts had been obtained, show that iron exhibits 

 a most remarkable and anomalous thermo-electric deportment at 

 a red heat. In his Bede Lecture* that eminent physicist points 

 out that " the cause of this is that while, as Sir W. Thomson dis- 

 covered, the specific heat of electricity in iron is negative at 

 ordinary temperatures, it becomes positive at some temperature 

 * Nature, June 12, 1873. 



