264 Mr. H. Tomlinson on raising Iron under Temporary 



which way the end B shall be jerked*. This experiment 

 was repeated with other specimens of iron and steel and with 

 similar results, except that on cooling, as with the torsional 

 stress and strain, the phenomenon occurred at a lower tempe- 

 rature than with the very soft iron wire. 



The question now arises, Is there any sudden molecular 

 change at a bright red heat even where there is no stress or 

 strain affecting the ivire? Professor Barrett brings forward 

 evidence in support of the view that there is, which I am 

 afraid is not conclusive. He ssljs : — " If, however, this 

 molecular change be entirely due to alteration in cohesion, 

 then the removal of the spring ought to cause the anomalous 

 behaviour to disappear. But it does not. Without the 

 spring, an iron wire can be seen by the naked eye to undergo 

 a momentary contraction during heating, and a momentary 

 and more palpable elongation during cooling f. Fixing one 

 end of the wire, and bending the other extremity at right 

 angles so that it may dip into a trough of mercury, and thus 

 preserve contact with the battery, both actions can be seen." 

 I have little doubt myself that the effects observed here were 

 really due to the stress produced by the weight of the ivire 

 itself. So far as my own experiments go, I have not been 

 able to detect any sudden change in the wire when sufficient 

 care is taken to prevent the weight of the wire itself from 

 producing an effect J. Nevertheless I am inclined to believe, 

 with Professor Barrett, that a sudden molecular change does 

 occur at the critical temperature even when the wire is quite 

 free from any mechanical stress or strain, though as yet expe- 

 rimental evidence is wanting to furnish sufficient evidence of 

 such a change. 



The Recalescence of Iron. 

 This curious phenomenon was, I believe, first discovered by 

 Professor Barrett, and mention is made of it in his paper, 

 already alluded to, in the following words : — " On September 

 12th I was examining the condition of the wire in a darkened 

 room, when a new and unexpected change revealed itself. 

 During the cooling of the wire it was found that just as it 

 reached a very dull red heat, a sudden accession of tempera- 

 ture occurred, so that it glowed once more with a bright red 



* That is, as to whether the effect of stress or that of strain shall 

 predominate. 



t The effect on cooling could always be got in my own experiments 

 also with greater distinctness than the effect on heating. — H. T. 



X It is difficult to realize, without actually putting the matter to the 

 test of experiment, what a very small amount of stress or strain suffices to 

 bring out the phenomenon. 



