THE MOLECULAR PROCESS m MAGNETIC INDUCTION. 267 



tlieir bt'iii,!;' tliiown into vibration. When tlie field i-s weak, heating 

 consequently assists magnetization, sometimes very greatly, by has- 

 tening tlie passage from stage (( to stage b of the magnetizing process. 

 And it is at least a conjecture worth consideration whether tlu^ sudden 

 loss of nmgnetic quality at a higher temperature is not due to the vi- 

 brations becoming so violent as to set the molecules spinning, when^ 

 of course, their polarity would be of no avail to produce magnetiza- 

 tion. We know at all events that when the change from the magnetic 

 to the non-magnetic state occurs, there is a ]>rofound molecular change, 

 and heat is absorbed which is given out again when the reverse change 

 takes place. In cooling from a red heat, the iron actually extends at 

 the moment when this change takes i)lace (as was shown by Gore), 

 and so much heat is given out that (as Barrett observed) it re-glows, 

 becoming brightly red, thongh just before the change it had cooled so 

 far as to be quite dull. [Experinu'nt, exhibiting retraction and re- 

 glow in cooling, shown by means of a long iron wire, heated to redness 

 by an electric cnrrent.] The changes which occur in iron and steel 

 about the temperature of redness are very complex, and I refer to this 

 as only one possible direction in which a key to them may be sought. 

 Perhai)S the full explanation belongs as much to chemistry as to physics. 



An interesting illustration of the use of these models has reached 

 me, only to-day, from Xew York. In a paper just published in the 

 Elect riad World (reprinted in the Ulectric tan for May 29, ISDl), IMr. 

 Arthur lloopes supi)orts the theory I have laid before you by giving 

 curves which show the connection exi)erimentally found by him 

 between the result polarity of a group of little pivoted magnets and 

 the strength of the magnetic field, when the field is apidied, removed, 

 reversed, and so on. I shall draw these curves on the screen, and 

 rough as they are, in consequence of the limited number of magnets, 

 you see that they succeed remarkably well m reproducing the features 

 Avhich we know the curves for solid iron to ])()ssess. 



It may, perhaps, be fairly claimed tliat the models whose behavior 

 we have been considering have a wider application in physics than to 

 merely elucidate magnetic i)rocesses. The molecules of bodies may 

 have polarity which is not magnetic at all — jxdarity, for instance, due 

 to static electrification — under Avhicli they group themselves in stable 

 forms, so that energy is dissipated whenever these are broken up and 

 re-arrangcd. When we strain a solid body beyond its limit of elasticity, 

 we expend work irre<;overab]y in overcoming, as it were, internal fric- 

 tion. What is this internal friction due to but the breaking and 

 making of molecular ties ? And if internal friction, why not also the 

 surface friction which causes work to be si)ent when one body rubs 

 upon another. In a highly suggestive passage of one of his writings,* 



* Encijclopmlia Brit., Niuth Ed., 1877, art., " Coustitutiou of Bodies," vol. vi, p. 

 313, 



