BRIDGMAN. — MERCURY UNDER PRESSURE. 353 



when exposed to purely hydrostatic pressure, should suffer anything 

 except mere change of volume without change of shape. If, however, 

 the substance be heterogeneous, so that the elastic constants vary from 

 point to point, then the application of hydrostatic pressure will be fol- 

 lowed by change of shape, yielding of one part at the expense of an- 

 other. If the pressure is high enough, it is conceivable that the elastic 

 limit may be exceeded in some places and not in others, resultingin 

 greatly exaggerated change of shape. Even if the elastic limit is not 

 exceeded, there are other effects which will produce irregular action, 

 such as elastic after-effects and hysteresis, and these effects become 

 rapidly more prominent at high pressures. It follows, therefore, that 

 the irregularities of compressibility determinations over a wide pres- 

 sure range are going to be more than proportionally greater than the 

 irregularities over a narrow range. 



All previous determinations of compressibility have been made under 

 fairly favorable conditions. The work to the highest pressures has 

 been done by Amagat, to 3000 atmos, in which he determined the 

 compressibility of the compressible liquids in a glass envelope. His 

 determinations of the compressibility of the most incompressible liquid, 

 mercury, were only to 50 atmos. The highest accurate work on mer- 

 cury seems to have been done by Richards to 500 atmos, a compar- 

 atively low pressure. Richards also used glass piezometers. The 

 highest previous work of any sort on mercury seems to have been done 

 by Carnazzi,^ to 3000 atmos, also with a glass piezometer. His accu- 

 racy was not such as to justify more than two figures in the final 

 result. 



In determining the compressibility of mercury to pressures as high 

 as 12,000 kgm., as it was desired to do in this paper, we have conjunc- 

 tion of the causes most unfavorable to accuracy ; low compressibility 

 of the liquid, and greatly exaggerated irregularity in the distortion of 

 the containing vessel. The employment of a glass containing vessel 

 would seem to be out of the question. In a previous paper,' the lin- 

 ear compressibility of glass has been determined directly to 6800 kgm. 

 Over this lower range, irregularities were found amounting to 4 per 

 cent. In using a glass piezometer, this irregularity would be magni- 

 fied by the fact that the compressibility of glass is high compared with 

 that of the mercury. The ideal substance is one which can be ob- 

 tained in a state of great homogeneity, and which at the same time has 

 as low a compressibility as possible. Steel has these properties in a 



" Camazzi, Nuov. Cim. (5), 5, 180-189 (1903). 

 ^ Bridgman, loc. cit. 



VOL. XLVII. — 23 



