l6 ELECTROMOTIVE FORCE OF IRON AND OCCLUDED HYDROGEN. 



cific gravity before and after compression. He first submitted to pressures 

 up to 10,000 atmospheres more than eighty solids of the most diverse chem- 

 ical nature, and found that powders weld together perfectly under pressure, 

 as we found with iron. In order to do this, he found it necessary to com- 

 press in a vacuum, because imprisoned air not only prevented contact of the 

 particles, but also tore them apart again when pressure was released. All 

 of his work was performed with the care which this precaution indicates. 

 The result of one investigation, entitled " L'elasticite parfaite des corps 

 chimiquement definis," is best given in his own words : 



La plupart des corps examines ont manifeste une petite augmentation permanente 

 de densite apres avoir subi une pression d'environ 20,000 atmospheres, mais, celle-ci 

 une fois realisee, ils ont resiste opionatrement a toute diminution permanente ulterieure 

 de leur volume. Pour tous la densite a atteint bientot mm maximum. La faible aug- 

 mentation de densite permanente observee n'a cependant jamais eu pour cause une 

 contraction reelle de la matiere. Toujours elle a ete due a l'ecrasement, par pression, 

 des cavites existant d'avance dans le corps examine, ou bien a la disparition de fissures 

 plus ou moins evidentes. En un mot, on s'est trouve en presence d'un fait accidentel, 

 ppuvant facilement amener une erreur, mais non d'un phenomene physique essentiel. 



Thus the present work on the electromotive force of iron subjected to 

 great stress confirms the very different work of Spring on densities. In so 

 far as these data show that when no change of specific gravity is effected by 

 a given application of energy, no change in the free energy of the system 

 results, they support the theory of compressible atoms. 



TENSION EXPERIMENTS. 



Although it thus appears to be impossible to produce permanent free energy 

 changes in pure iron through pressure alone, it appeared possible that 

 measurements of electromotive force made while the metal was under tensile 

 stress might show varying values, because tension produces marked physical 

 changes in the metal. These changes are as follows : ( 1 ) A strain which 

 immediately disappears on release of the tension, inside the elastic limit. 



(2) Beyond the elastic limit a permanent strain, with increasing stress 

 needed to produce equal increment of strain, up to the ultimate resistance. 



(3) Beyond the ultimate resistance, with ductile metals, there is a region 

 before the actual break occurs, where, for increase of strain, decreasing 

 stress is required, until failure takes place. It seemed reasonable to expect 

 that such decided and sudden changes in the cohesion would be accompanied 

 by free-energy changes. 



While under stress, or after a " permanent set " has been produced by 

 strain beyond the elastic limit, the iron is in unstable equilibrium. Sufficient 

 proof that it is so within the elastic limit lies in the fact that it returns of 



