THE EFFECT OF HIGH PRESSURE. 13 



nomenon seems to be the conclusion that the exposure to the air results in 

 the formation of an exceedingly thin coating of oxide or at least of adsorbed 

 oxygen, and that when this coating is removed, by solution or reduction, 

 the true electromotive force is obtained. 14 



Accordingly, the iron to be used in the succeeding experiments was sealed 

 in hydrogen. The tubes were broken open just before the samples were 

 to be used, and their electromotive forces were measured as soon as possible 

 afterwards. In this way the exposure to air was as short as possible and 

 the theory was confirmed, for they were found to attain their maximum 

 values very promptly. 



With these samples a more careful study of the effect of pressure was 

 begun. A number of preliminary experiments were made with pressures 

 produced by 30,000 and 60,000 pounds. These pressures were quite suffi- 

 cient to cause cold-welding of the particles of the porous iron, giving firm, 

 hard plates with bright metallic surfaces. The area of these surfaces was 

 seldom more than the half of a square centimeter, hence the pressures were 

 approximately 30,000 to 60,000 kilograms per square centimeter. The 

 compression was done between two smooth plates of steel in a large testing- 

 machine in the Engineering Department of Harvard University. The 

 pressure was great enough to press the soft iron into the steel plates, leaving 

 distinct indentations. The iron was handled only with iron pincers. Every- 

 thing with which it came in contact was kept scrupulously clean. The edges 

 of the compressed pieces being cracked and split, they were embedded in 

 soft paraffin before immersion in the ferrous sulphate solution, leaving 

 exposed for measurement only the center, which had certainly borne the full 

 effect of the pressure. 



It appeared that iron thus treated gave results in no way different from 

 the spongy pieces before compression, except that the compact specimens 

 were slower in reaching their maxima. When reached, these maxima were 

 essentially identical with those of the porous specimens. 



Nevertheless, it seemed worth while to carry the matter further. Through 

 the kindness of Lieutenant-Colonel J. G. Butler in charge, it was possible 

 to subject other portions of pure spongy iron to still greater pressure on the 

 magnificent testing machine at the United States Arsenal at Watertown, 

 Massachusetts. This machine is capable of not only exerting a weight-effect 

 of 1,000,000 pounds, but of measuring with great accuracy the magnitude 

 of the effect it exerts. Two special blocks were made between which to 



"This experience, and the conclusion derived from it, agrees with that of Mutti- 

 mann and Fraunberger (Sitzber. Akad. der Wiss. Miinchen, 34, 201) (1904). Their 

 investigation was published after this part of our research had been completed, and 

 was wholly independent. 



