VARYING THE TEMPERATURE OF IGNITION. 23 



should be noted that steel alone, not pure iron, was experimented upon ; that 

 any speed of reaction in which two phases are involved is complicated by 

 many disturbing factors ; and that the tendency of iron to occlude hydrogen, 

 discussed in a later section of this paper, must have added another 

 complication. 23 



Because our results obtained by applying both compressing and distending 

 stresses upon pure iron had been chiefly negative, indicating very slight 

 changes of free energy for large stresses, attention was now directed to the 

 change of the other conditions of experiment, with the hope of tracing to 

 these other conditions the fairly large changes of electromotive force actually 

 to be observed in specimens of pure iron prepared in different ways. 



THE EFFECT OF VARYING THE TEMPERATURE OF IGNITION. 



It will be noticed on studying tables 2 and 3 that the electromotive forces 

 of cells made from different samples of iron which were chemically alike 

 varied between such wide limits as 0.780 to 0.794 volt, a range far beyond 

 the limits of experimental error. 



In tracing these differences back to their fundamental cause, the first clue 

 was furnished by the different degrees of cohesion or compactness exhibited 

 by the several specimens. When the four pieces of porous iron, the results 

 from which are given in table 3, were removed from the cells at the comple- 

 tion of the electrochemical experiments, they were examined with regard to 

 their cohesion by the simple method of resistance to pressure and to fracture 

 between the fingers. It was thus found that No. 2 iron felt softest, No. 3 

 the hardest and firmest, and Nos. 1 and 4 somewhere between. Reference 

 to the table shows that No. 2 had the highest electromotive force and No. 3 

 the lowest ; hence a more powdery structure evidently went with greater 

 tendency to dissolve. Repetition with other samples confirmed this view. 



What, now, could be the cause of this difference in compactness of the dif- 

 ferent samples? It is well known that many substances, on being heated, 

 begin to cohere at temperatures far below their melting-points, exhibiting 

 the shrinkage commonly called " sintering." It is also well known that this 

 effect becomes more and more noticeable as the melting-point is approached. 

 Probably, then, these different samples had been subjected to different tem- 

 peratures during their reduction. This relationship was easily confirmed 

 by further experiment. Iron reduced at high temperatures was found to be 

 far firmer in structure than that reduced at low temperatures. In order thus 

 to find the highest potential attainable in this way, iron should obviously be 



28 In this connection attention is called to the interesting work of C. S. Burgess on 

 the effect of impurities on the rate of solution, read at the recent meeting of the 

 American Electrochemical Society, May, 1906. 



