BRIDGMAN. — MERCURY UNDER PRESSURE. 367 



0.06559. A repetition of the former experiment with the tool steel cylin- 

 der gave O.OeSGS at 20°. It is to be noticed that this value is higher 

 than that previously found, and in the direction toward that shown by 

 the nickel steel cylinder. During the repetition of the experiment the 

 tool steel cylinder also showed less hysteresis than in the early deter- 

 minations, probably due to the gradual disappearance of the heterogen- 

 eity introduced by the previous high pressure during the two and one 

 half years for which the cylinder had been resting. During this last 

 set of determinations the pressure was never raised above 4000 kgm., 

 which is below the elastic limit of the steel. 



Whether the true explanation of the discrepancies is to be found in 

 the warping of the cross-section of the cylinder or not, the fact seems 

 to be that when the elastic limit of the material is exceeded there is 

 resulting hysteresis in the elongation, and that this hysteresis is accom- 

 panied by low values for the compressibility, the greater the hysteresis, 

 the lower the values. It seems to be justifiable therefore, to accept as 

 most probably correct the values given by this new determination with 

 the nickel steel cylinder. The values given above and the temperature 

 coefficient deduced from them were used in the following computations. 



It is to be noticed that the elongation of the nickel steel cylinder 

 under internal pressure, which has appeared in the above as a correc- 

 tion factor, is sufficient to give the cubical compressibility of the 

 nickel steel. This is because, as was pointed out by Mallock ^"^ in 

 1904, the elongation of a hollow closed cylinder under internal pres- 

 sure involves only one elastic constant, the cubic compressibility. 

 This gives for the cubic compressibility 6.2 X 10"'' at 20°. The 

 results are not accurate enough to give the temperature coefficient by 

 this method. The high value of this result compared with that by 

 the direct observation of linear compressibility (62 against 58), is prob- 

 ably due to the difference in the materials, the latter value being for 

 an almost pure iron, while the former is for a nickel-chrome alloy with 

 little or no carbon. (Krupp's trade number for this steel is E. F. 60.0.) 

 It was shown in the previous paper that carbon alone makes very little 

 difference in the result. 



This same method, the elongation of a hollow cylinder under inter- 

 nal pressure, has been recently used by Griineisen ^^ in determining 

 the cubic compressibility of various metals at different temperatures. 

 His results for two different specimens of iron at 18° varied from 

 58. X 10"^ to 63. X 10~^ The temperature effect on this last speci- 



" Mallock, Proc. Roy. Soc. Lon., 74, 50 (1904). 

 " Gruneisen, Ami. Phys., 33, 1239-1274 (1910). 



