in Iron and Steel at a Bright-Red Heat. 437 



of the various physical properties suddenly change at that 

 critical temperature. 



I will now return to the subject of Mr. Tomlinson's 

 paper ; and as I have independently made observations on 

 similar points, I will give my view of the case, as applied to 

 Mr. Tomlinson's experiments. In Experiment I. a wire was 

 heated under torsional stress, and when a certain tempera- 

 ture was reached the rigidity gave way somewhat suddenly 

 and the wire was permanently twisted. " The slight check in 

 the rate at which the wire was permanently twisting "is to 

 be attributed to a similar check in the rate of decrease of 

 rigidity at the " darkening/'' The wire was allowed to cool ; 

 the rigidity increases until the reglow takes place, and this 

 involves a sudden rise in temperature with corresponding 

 fall in rigidity, and the wire twists farther. 



I have been nonplussed by what appears to be an incon- 

 sistency in the results recorded in Experiments I. and II. 

 In Experiment I. the stress produces permanent strain ; in 

 II. a very feeble stress (magnet deflected through 10°) pro- 

 duces a permanent strain ; but an intermediate stress (magnet 

 deflected through 90°) is not stated to have produced anything 

 but a temporary effect at the higher temperature ; and this 

 effect — untwisting during the heating of the wire — is such as 

 to overcome the external stress, as if we had to deal with 

 some instability. I hesitate to attempt an explanation of the 

 first part of Experiment II. ; the second part admits of an 

 attempt, suggesting further tests. 



In Experiment II. (second part), then, we have to deal with 

 the case of a wire under very feeble external stress without 

 permanent strain ; as the temperature rises there is a sudden 

 untwist, that is, the needle is deflected further from the 

 meridian, and also permanent twist ; and as the wire cools, 

 there is a sudden temporary twist at nearly the same tempera- 

 ture. I think it possible to explain the untwist by attributing 

 it to a pause in the permanent twisting, resulting in a swing 

 of the magnet temporarily from the meridian ; the pause 

 being due to the fact that the " darkening " is taking place, 

 and with it a pause in the decrease of rigidity. It would be 

 interesting to repeat this experiment with a view to testing 

 how far all the movements of the needle actually represent 

 stresses in the wire. 



Experiments IV. and V. are of great interest as showing 

 both the effect of permanent stress (I leave out of the case 

 the magnetic effects observed), and the failure of a very 

 common method of annealing. 



With permanent torsional strain are introduced internal 



