THE PERFECT ELASTICITY OF PURE IRON. 15 



more to the fact that the compressed pieces at first yielded too low an 

 electromotive force, although the value afterwards rose to equal that of the 

 uncompressed pieces. An explanation for these low values is to be found 

 in the idea already mentioned on page 13, there advanced to explain the lag 

 of porous iron which had been kept for some time in a desiccator. It is 

 that an exceedingly thin coating of oxide is formed on the iron by exposure 

 to the air and thus the true electromotive force is masked until this coating 

 is dissolved. Most of the samples of porous iron had been kept a whole 

 day in a desiccator after reduction before immersion, while sample 14 in 

 table 3 had been kept only three hours. The difference in the speed of 

 attaining the maximum value is very marked. This theory not only seems 

 to account for the effect of long exposure to air, but also offers a simple 

 explanation for the slightly lower initial potential of the compressed samples. 

 The length of time during which the compressed pieces were exposed to 

 the air before measurement was exactly the same as that of the parallel 

 porous ones, but the former were inevitably heated somewhat in the act of 

 compression, and thus an oxide coating might well have been formed which 

 would equal in effect one of long, slow formation in a desiccator. The 

 theory probably also offers the most satisfactory explanation of the exag- 

 gerated deficiency exhibited by No. 16 in table 3. The blocks between 

 which No. 16 was compressed had been three times subjected to such enor- 

 mous pressures that they were beginning to fail, and this fourth and last 

 time the blocks split with bright sparks and loud reports, and threw small 

 pieces in all directions. Under such treatment the blocks undoubtedly 

 became warm, and it is quite to be expected that this last sample should 

 possess the heaviest and firmest coating of oxide. 



Thus it is fairly safe to conclude that even such enormous pressures as 

 300,000 kilograms per square centimeter produce no permanent change of 

 free energy in iron. 



It is interesting to compare this constancy with the behavior of other 

 properties of metals on subjection to great stress. Since this work was 

 done, Dr. Stull and one of us has found, in the work already referred to, 

 that after subjection to pressures up to 500 atmospheres all of thirty-odd 

 elements examined regain precisely the original volume. The method used 

 is unique and allows of the detection of volume changes of 0.000,01 milliliter 

 in a sample of 10 milliliters. 



After this work was done we became acquainted with two pertinent 

 papers by W. Spring, 16 in which he proves the same fact by still another 

 method, the one, in fact, which would most naturally be adopted if that 

 were the main object of the research, namely, the determining of the spe- 



16 Bull. Acad. Roy. de Belg. (3), G, 509 (1883); Ibid. (1903), p. 1066. 



