PART I. THE ELECTROMOTIVE FORCE OF PURE IRON 

 UNDER VARYING CONDITIONS. 



When in a definite state of internal equilibrium, the electromotive force of 

 a metal immersed as a reversible electrode in a definite solution of one of its 

 salts must have a perfectly definite value under given conditions. A change 

 in the internal state must of necessity change this electromotive force, unless 

 the temperature happens to be exactly at the transition point. For this 

 reason it seemed not improbable that the measurement of the electromotive 

 force of iron which had been subjected to great stress would afford the most 

 convenient method of determining whether or not this stress had affected a 

 permanent alteration in the internal equilibrium of the metal, a " permanent 

 set," as it is sometimes called, or an extraordinary condition of metastable 

 equilibrium. 



A solid may be subjected to a variety of stresses. The first which it 

 seemed desirable to study was direct pressure, in order to determine if this 

 pressure, by causing closer internal structure, diminishes the outward or 

 dissolving tendency as measured by electromotive force, or if, on the other 

 hand, the substance, when released from the pressure, returns at once to 

 essentially the state in which it was before the pressure was applied. Experi- 

 ment alone could decide between these alternatives. 



It was desirable also to test the possibly opposite effect of a negative 

 pressure, a distending tendency, such as the effect of stretching a wire to 

 the breaking-point. This effect was also made the subject of experiments 

 recorded below. 



During these tests it was found that the temperature used in the reduction 

 of the iron from its oxide caused a very important effect on the dissolving 

 tendency, and therefore this matter also had to be considered in a special 

 series of experiments. 



PREPARATION OF MATERIAL. 



In any research with iron it is of primary importance that the substance 

 experimented upon should be pure free from carbon, silicon, sulphur, 

 phosphorus, manganese, and, as the following work testifies, from hydrogen 

 also. 



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