of the Physical Properties of Iron. 249 



When the wire has fully " accommodated **.' itself, a very 

 small cause will disturb the accommodation : a mechanical 

 shock, a change of load, a slight rise or fall of tempera- 

 ture, or even rotation of the molecules by'magnetic stress, will 

 necessitate fresh oscillations before the friction again reaches 

 its minimum. Consequently, if a wire be raised to 100° C, 

 and be then cooled again rather quickly, it does not imme- 

 diately regain the accommodation which it had before heating. 

 The time taken by the wire to reaccommodate itself when 

 the accommodation has been disturbed by change of tem- 

 perature, depends considerably upon the direction of the 

 change. Thus, when the wire is raised from the temperature 

 of the room to 100° C, the reaccommodation is effected in a 

 very much shorter time than when the accommodation has 

 been disturbed by lowering the temperature from 100° C. to 

 the temperature of the room. 



I am inclined to regard both the temporary and permanent 

 alterations of the internal friction of iron, which are produced 

 by rise of temperature not exceeding 100° C, to be partly due 

 to mere agitation of the molecules, but the permanent effects 

 do not seem to be entirely due to molecular agitation ; for the 

 maintaining of the temperature at 100 c C. for some time does 

 not bring down the friction anything like so much as repeated 

 heating and cooling. It would seem that the slow shifting 

 backwards and forwards of the molecules induced by the last 

 process is in a great measure to be credited with the permanent 

 diminution of friction, in the same manner that the shifting 

 backwards and forwards of the molecules caused by torsional 

 oscillation has been shown to produce permanent diminution. 



The Longitudinal and Torsional Elasticity of Iron. 



It might readily be imagined that since the internal friction 

 of iron is so considerably altered by change of temperature, 

 the elasticity would be correspondingly affected. This, how- 

 ever is not so ; both the torsional and longitudinal elasticity 

 of iron are affected by raising the temperature to 100° C, but 

 not nearly to the same extent as the internal friction. Thus 

 an annealed iron wire, when suspended ready for torsional 

 vibration, was heated slightly by passing the flame of a 

 Bunsen's burner rather quickly up and down it several times. 

 The time of vibration before heating was 1*154 second ; and in 

 5 minutes, 35 minutes, and 245 minutes after cooling was 

 1*147, 1*142, and 1*136 second respectively. Here we have 

 a small, though distinct, permanent increase of elasticity ; and 

 it will be noticed that time is an important element in the 

 amount of increase. Again, by a very carefully conducted 

 set of observations, I have shown that the torsional elasticity 



